Fintech Software Development
Fintech Software Development That Gets The Ledger Right First
In fintech the interface is the easy part. The hard part is money that has to add up after a retry, a balance two systems agree on, and a record an auditor can follow. We design the ledger, the idempotency and the reconciliation before anyone styles a screen.
Triple Protection Guarantee
- US-Registered Entity
- Signed IP Assignment
- Senior Engineers Only
Triple Protection Guarantee:
- US-Registered Entity
- Signed IP Assignment
- Senior Engineers Only
Years In Business
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Engineers On Staff
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Avg. Engineer Exp.
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Where Fintech Bites
Money Modelling
Never Float
Senior Engineers
Vetted Only
Timezone Overlap
Live Hours
Safe Retries
On Each Write
Delaware LLC
US Entity
Data Security
Access Rules
Quality Control
Code Review
Reconciliation Runs
Scheduled
Audit Trails
Immutable
IP Assignment
Signed
Delivery Overlap
Fixed
Hours
Code & IP Owner
You
Trusted By Startups
What Fintech Software Development Really Means
Fintech software development is building systems where a wrong number is not a bug but a loss. That changes the engineering. Money is stored in minor units rather than floating point, every write that moves value is idempotent so a retry cannot pay twice, balances are derived from an append-only ledger instead of being edited, and reconciliation runs against the source of truth on a schedule rather than when somebody notices a gap.
Stallyons is registered in Delaware as a US company, and our engineers work a time-zone window agreed before the project starts, with four-plus hours of daily overlap. You sign a US contract, run diligence on a US entity and pay one US invoice. Behind the work sits twelve-plus years of delivery across six continents and around thirty-five engineers, averaging four-plus years of production experience. Founders, operations leads and product owners come to us with money movement that has to be provably correct.
What Fintech Delivery Covers
A ledger before a screen: append-only entries, balances derived rather than stored, and amounts held in minor units, so the number a customer sees and the number finance reports are the same number.
Idempotency on every value-moving call: request keys, safe retries and exactly-once effects, because payment infrastructure fails midway and the correct response to a timeout is never a second charge.
Reconciliation designed in, not added: scheduled runs against processor files and bank statements, with breaks raised as work items rather than discovered at month end by a spreadsheet.
Onboarding that carries its evidence: identity checks, document capture and screening results stored with the decision and the reason, so a later review does not become an excavation.
Card data kept out of your estate: tokenisation and hosted fields so raw card numbers never touch servers you own, which is the cheapest way to keep PCI scope small.
One accountable vendor: one contract, one invoice and one entity for legal and finance to run diligence on, instead of a spread of contractors across four jurisdictions.
Why Finance Teams Choose A Fintech-Specialist Partner
- Ask how they represent money. If the answer involves a floating point type, or they have never had to explain rounding on a split payment, you will find out the hard way.
- Ask what happens when a payment call times out. You want request keys and exactly-once effects, not a support process for refunding the customers charged twice.
- Ask when reconciliation gets built. Teams that add it after launch are the same teams explaining a six-figure break to finance a quarter later.
- Ask where card data lands. If any raw card number reaches a server you own, your compliance surface just grew, and the decision that caused it was an engineering one.
- Check the exit before you need it. Repositories, cloud accounts and processor credentials sit in your own name from day one, so leaving is administration, not a negotiation.
- Insist on a written handover: decisions, open threads and known issues documented as work happens, so whoever picks it up next reads instead of guesses.
How A Fintech Project Starts With Us
Every project starts with a free 45-minute scoping session. No slide deck, no sales script. You bring the money flows, the partners and the controls you answer to; you leave with a ledger model, a scope and a timeline.
We are selective about new projects and cap how many we run at once, because the scoping is the product. If your product needs a regulated capability we cannot deliver properly, we will say so first.
Why Clients Choose Us
Full
Written IP Transfer
USA
Contract Entity
Yours
Ledger & Accounts
Named
Delivery Lead
Ready to build money movement that always adds up?
What We Build In Fintech
Fintech Software Development We Deliver
Fintech products differ at the edges and rhyme at the core: a ledger, a set of integrations and a control that has to hold. These are the builds we deliver most often, each scoped against your own money flows.
Payment Integrations
Gateways, rails, reconciliation
End-to-End
Ledger Systems
Double-entry, balances, audit
Kept Immutable
Onboarding & KYC
Identity checks, documents, screening
Verified
Banking Integrations
Open banking, files, webhooks
Live In Production
Fraud Rules
Scoring, limits, review queues
Under Review
Reporting & Recon
Statements, exports, evidence
Balanced
Customer & Ops Portals
Accounts, limits, admin tools
Served
QA & Test Automation
Manual, automated, regression, sign-off
Sign-Off
Core Modernization
Rewrites, migrations, replatforms
Future-Proof
Support & Maintenance
Monitoring, fixes, releases, cover
Kept Running
Not sure where your money flow breaks? Let's map it out.
Common Challenges
Why Do Fintech Builds Go Wrong?
Six failure patterns behind almost every fintech rebuild we are asked to quote. All six are decided in week one.
Floats For Money
01
Amounts stored as floating point look fine in testing and drift by fractions of a cent at volume. The gap shows up as a reconciliation break nobody can explain, months after the code shipped.
Retries Charge Twice
02
A payment call times out, the client retries, and the effect lands twice because nothing carried a request key. Customers find this defect before your monitoring does.
Balances Overwritten
03
Balances stored as an editable column instead of derived from entries. Once a value has been overwritten, no query can reconstruct how the account reached it.
Recon Left Late
04
Reconciliation is scheduled for after launch, so processor files go unchecked for a quarter. The first real run surfaces a backlog of breaks with no owner and no audit trail.
Card Data In Your Estate
05
One convenient form posts a raw card number to your own server, and your compliance surface now covers every system that log line touches on its way to storage.
Controls Bolted On Last
06
Approval limits and segregation of duties arrive as a late request, so they are enforced in the interface only, and anyone with API access can step around them.
Recognise a few of these? Let's do it properly.
Our Fintech Services
6 Fintech Software Development Services
Six ways to buy fintech delivery from one accountable vendor. Run one, or run several in parallel under a single contract.
Payment Platform Development
01
Checkout, gateway and rail integrations built with request keys, webhook handling and retry policy, so the effect of every call is defined before the first payment moves.
Ledger Engineering
02
An append-only double-entry ledger with derived balances, minor-unit amounts and defined rounding, built as the source of truth every other service in the product reads from.
KYC & Onboarding Flows
03
Identity capture, document upload, screening callbacks and manual review queues, with the decision and its evidence stored against the customer record.
Core Banking Integration
04
Connecting the core, the processor and the file exchanges that will outlive this project, through our API development work, in stages you approve.
Risk, Fraud & Controls
05
Scoring rules, velocity limits, review queues, approval thresholds and segregation of duties, enforced in the service layer rather than only in the interface.
Cloud, Support & Monitoring
06
Hosting, pipelines and alerting on our cloud app development stack, watching settlement windows, failed-webhook queues and the nightly reconciliation job.
Not sure which fintech service you need? Let's scope it together.
Why Choose Us
What Makes Our Fintech Software Development Different
The details that decide whether a fintech build survives its first audit and its first spike.
A US Legal Entity
01
Stallyons is registered in Delaware. Your contract, your invoice and your legal recourse sit with a US company, not an unknown one.
Money Modelled Right
02
Minor units, defined rounding, an append-only ledger and derived balances, agreed in scoping because none of the four is cheap to change later.
Overlap You Set
03
You choose the hours we share with your working day, and stand-ups, reviews and escalations all happen inside that window.
Aligned To Standards
04
We build HIPAA, SOC 2, GDPR and PCI-DSS-aligned, and in fintech that starts with keeping raw card data out of systems you have to defend.
Reviewed Code
05
Every merge is reviewed against an agreed definition of done, on your board, where you can read it yourself.
One Contract
06
One contract covers the engagement, so procurement, legal and finance each deal with a single named counterparty.
Ready to see what a fintech build looks like here?
Our Process
From First Call To Fintech Delivery In Six Steps
A delivery process built to settle the ledger and the controls before anyone writes code.
Discovery
Understand the money flows, partners and volumes
Scoping
Agree the ledger model, scope and cost
Design
Entries, limits and approval paths signed off
Contracting
NDA, IP assignment, access and onboarding
Deliver
Work on your board, reviewed on merge
Launch & Settle
Go live, then widen limits and volume
Want to see how this maps to your roadmap?
Technology Stack
The Stack Behind Our Fintech Software Builds
Boring, durable technology chosen because money systems outlive frameworks: backend, data, cloud.
Web & Backend
Java Services
Node.js APIs
Python / Django
.NET Core 8
TypeScript
Ledgers & Payment Rails
Double Entry
Idempotent Writes
Reconciling
Payouts
Multi-Currency FX
Risk & Identity
KYC Onboarding
AML Screening
Fraud Rule Engines
Audit Logging
Strong Authentication
Data & Records
PostgreSQL Core
MongoDB Docs
Event Logs
Warehousing & BI
Retention Policy
Cloud & DevOps
AWS / GCP / Azure
Docker / K8s
Terraform / IaC
GitHub Actions / CI
Datadog Monitoring
Segments We Serve
Fintech Software Development Across Finance Segments
Eight corners of finance with different rulebooks and the same requirement: the numbers have to hold.
Payments & Gateways
Checkout, rails and settlement
Digital Banking & Wallets
Accounts, cards and transfers
Lending & Credit Ops
Origination, scoring, servicing
WealthTech & Funds
Portfolios, orders, statements
Insurance & InsurTech
Quotes, policies, claims flow
RegTech & Supervision
Rules, reporting, evidence
Digital Asset Rails
Custody, ledgers, transfers
B2B Finance Operations
AP, AR, treasury, invoicing
Working in another sector? See all industries we serve.
How We Compare
Fintech Software Build vs Platform vs Freelance
An honest look at your four delivery options.
| Capability | Generalist Dev Shop | Payments Platform Alone | Freelance Team | Stallyons Technologies |
|---|---|---|---|---|
| How money is represented | Whatever the ORM gives | Minor units | ✕ Floating point | Minor units, defined rounding |
| Retry safety on value moves | Added after an incident | Their side only | ✕ Double charges | Request keys end to end |
| Ledger & derived balances | Editable columns | Their ledger, not yours | ✕ No ledger | Append-only, double entry |
| Reconciliation | Phase two | Reports to export | ✕ Spreadsheets | Scheduled, breaks as work items |
| KYC evidence retained | Decision only | Inside their console | ✕ Not retained | Decision plus its evidence |
| Keeping card data out of scope | Sometimes | Hosted fields | ✕ Raw card posts | Tokenised, never on your servers |
| Contracting entity & ownership | Varies | Platform terms | ✕ Marketplace terms | US-registered LLC, your accounts |
See the difference for yourself
Complete Engagement
Everything Included In A Fintech Software Build
From Scoping to Contracting to Delivery, One Vendor
Here's everything included when you build your fintech product here:
One Fintech Build Price: No Hidden Fees, No Surprises.
Every fintech engagement includes all eight components above. One contract, one senior team, one predictable cost, and no vendor sprawl.
🔒 No obligation. We'll deliver a detailed proposal within 48 hours.
Plus, Get These Free Bonuses
Free Ledger Review
A written read on how your product represents money, where a retry could pay twice, and which reconciliation you are not running yet. Yours to keep.
Included Free
Delivery Plan & Estimate
A phased delivery plan with scope, milestones, a stack recommendation and a transparent, itemized estimate for the engagement.
Included Free
Free Vendor Checklist
The questions we would ask any fintech team about ledgers, idempotency, reconciliation and card data, so you can test us on them too.
Included Free
Risk-Free Partnership
Our Fintech Delivery Promise
We stand behind every engagement with commitments that protect your investment.
01
Scope Agreed First
Scope, model, working hours and cost structure are written down and agreed before contracting, so nothing is discovered later.
02
Built to Last
Senior developers, code review, automated tests, security and accessibility audits, and clean, documented code you fully own.
03
IP And Access Protected
NDA and IP assignment are signed before access, permissions are scoped per person, and your accounts stay under your control.
Start your fintech build with confidence, backed by our Triple Protection Guarantee.
Track Record
Engagements That Ship, Scale, and Compound
500+
Projects Delivered
29+
Service Categories
81%
Repeat Client Rate
4.9 ★
Clutch Rating
"We came to Stallyons after burning two years and four vendors on a multi-platform launch that kept slipping. They scoped it end-to-end — web app, iOS, Android, an AI summarization layer, and a Shopify integration — and shipped it in 22 weeks. One team, one budget, one quality bar. We've handed them three more engagements since."
Mark Sawyer
CEO/Founder
PlatinumLED
"Stallyons rebuilt our customer-facing portal, integrated three legacy systems, shipped an AI document analysis pipeline, and brought our compliance posture to SOC 2 — all under one engagement. The senior engineers on the team have shipped at companies five times our size. It's the best vendor decision we've made in a decade."
Mark Sawyer
CEO/Founder
PlatinumLED
FAQ
Frequently Asked Fintech Software Questions
Fintech software development is building systems that move, hold or account for money, where correctness is the product. In practice it means an append-only ledger with derived balances, amounts stored in minor units with defined rounding, idempotent handling of every value-moving call so a retry cannot pay twice, reconciliation against processor and bank files on a schedule, and an audit trail a reviewer can follow without help.
A gateway moves a payment and tells you what happened to it. A fintech platform is everything around that: who the customer is and how they were verified, what the balance means, which limits applied, how a failed webhook gets retried, what reconciliation says at the close of each day, and what an auditor sees. Gateways are a component you integrate. The platform is the part you own, and it is where the engineering effort lives.
Cost follows the number of integrations, how much of the ledger and control surface has to be built rather than bought, and how strict the reporting burden is, so a figure before scoping is guesswork. What moves it most is integration count. We scope first, then price, and itemise what each phase covers so you can cut before you commit.
It depends on the scope, and we put a timeline in writing before contracting rather than quoting a stock number. What changes the answer most is how many partners you integrate and how quickly they respond. We deliver in phases, so a narrow flow with a working ledger and reconciliation can go live while the wider product is still being built.
You do, from the first commit. NDA and IP assignment are signed before anyone gets repository access, with no licence-back and no shared ownership. Repositories, cloud accounts, processor credentials and documentation live in your own accounts from day one, so there is nothing to negotiate later.
By keeping raw card numbers out of infrastructure you own. Hosted fields and tokenisation mean the card reaches your provider directly and your systems hold a token, which keeps the surface you have to defend small. We build HIPAA, SOC 2, GDPR and PCI-DSS-aligned, and we will say plainly which controls remain yours.
Usually, and we read the interface before proposing anything. Cores with an API become integrations; older ones are often reached through scheduled file exchanges, and we will flag where that is fragile. Migration happens in stages, with the existing core running until the new path has earned the switch.
Every value-moving call carries a request key, and the handler is written so repeating the same key returns the original result rather than performing the action again. Webhooks are treated as unreliable and replayable. Reconciliation then runs against the processor’s own records, so a double effect is caught by the system, not by a customer.
Still have questions? Let's talk.
Schedule an appointment with us today!
Ready To Build Fintech That Always Adds Up?
Get a free consultation. We'll walk your money flows, model the ledger behind them, and send a written proposal.
