A replayed webhook or a rounding error is enough to lose track of other people’s cash; here you learn to make that impossible.
You can ship a checkout page in a weekend. Then a customer double-clicks Pay, the webhook arrives twice, a refund lands before the charge settles, and your balances quietly stop matching the bank’s. This course starts where the payment button ends. First, how money physically moves: card rails, ACH (the US bank-transfer network), SEPA, UPI, and instant-payment schemes — and what “settled” actually means on each. Then gateway integration the way production demands it: idempotency keys, signed webhooks, retries, daily reconciliation, and the full lifecycle of refunds, disputes, and chargebacks. Next comes the accounting spine — double-entry bookkeeping from first principles, then an append-only ledger (the immutable log of every debit and credit) with materialized balances, three-way matching, and a period close that survives an audit. Finally the rules: KYC (Know Your Customer) identity checks, AML (anti-money-laundering) monitoring, sanctions screening, tax invoicing mandates, and sponsor-bank BaaS (banking-as-a-service) architecture. Five capstones — IBAN rails, a Stripe integration, a ledger service, a KYC/AML pipeline, and a multi-tenant wallet platform — are the proof. Regulatory material here is educational. It does not certify you to operate a regulated financial product in any jurisdiction; work with qualified legal and compliance counsel before you launch.
Built by Lakshya Kumar
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Idempotency keys, signed webhooks and daily reconciliation — the difference between a demo checkout and one you can trust.
Refund flows, evidence packets and reserve holdbacks decide whether disputes are a cost of doing business or the end of it.
Debits, credits and the balance equation from scratch, so every cent your product moves has two places it must show up.
Materialized balances, sagas and three-way reconciliation keep a hot ledger fast without ever letting history be rewritten.
Sanctions screening, transaction monitoring and suspicious-activity reporting, built the way a regulator expects to see them.
Document checks, liveness and business ownership verification tuned so real customers get through and fraudsters do not.
Gap-free numbering, tax rules and e-invoicing mandates turn a rendering task into the record revenue authorities audit.
Settlement timing, sponsor-bank partnerships and multi-tenant isolation — what it takes to move funds on someone else’s charter.
Velocity rules, device fingerprints and models that catch account takeovers in milliseconds, without blocking good customers.
Complete all modules, then submit the required number of capstone projects. Each must earn a passing rating from an admin reviewer.
Build a working virtual-account system that generates compliant IBANs (with correct ISO 13616 check digits for at least 3 EU country codes), maintains a per-IBAN virtual account ledger, and simulates SEPA Instant settlement against a mock partner bank. Include: IBAN validation, account creation, deposit/withdraw via simulated rails, real-time balance updates, and a partner-bank reconciliation report. Demonstrate the full happy path: create account → receive simulated SEPA Instant payment → see balance → withdraw to external IBAN.
I am learning to build money-moving fintech products — payment rails (ACH, wire, SEPA, UPI, FedNow), payment gateway integration (Stripe, Razorpay), refunds and chargebacks, double-entry bookkeeping, financial ledger engineering, invoicing and tax compliance, KYC/AML regulations, identity verification, BaaS architecture, and fraud detection. Help me understand the actual mechanics with reference to real regulations, real systems, and real production patterns.
Build a complete Stripe checkout integration: idempotent PaymentIntent creation, client-side payment collection via Stripe Elements, webhook handler with signature verification + deduplication, automated retry logic for transient errors, refunds with webhook handling, and chargeback evidence submission. Include daily reconciliation against Stripe's balance API. Cover the happy path AND failure modes: declined cards, 3DS challenges, partial captures, full refunds, chargebacks. Must work end-to-end on Stripe test mode.
Implement a production-grade double-entry ledger as a REST API. Required: journal entries with sum-of-debits=credits constraint at database level, append-only design, sub-ledger pattern (per-customer accounts under aggregate G/L), idempotency via external_ref, period close with backdating prevention, three-way reconciliation against mock processor + bank, and trial balance + P&L + Balance Sheet generation. Stress-test with 10k concurrent transactions; verify zero drift.
Build an end-to-end onboarding + monitoring system. Required: KYC integration with a real provider's sandbox (Onfido, Persona, Sumsub, or Stripe Identity), sanctions screening against a synthetic OFAC SDN list (100+ names), risk scoring with documented inputs, transaction monitoring with 5+ AML rules (structuring, velocity, country risk, etc.), and a compliance-officer dashboard for alert review. Test with 1000 simulated users + 10k simulated transactions; document the false-positive rate.
Build a multi-tenant wallet platform that abstracts a sponsor-bank-style architecture. Required: tenant onboarding with per-tenant KYC config, per-tenant data isolation via Postgres row-level security, virtual accounts with multi-currency support (at least USD and EUR with FX), simulated rails (ACH, FedNow, wire) with correct settlement times, working capital tracker, and a tenant-facing dashboard. Demonstrate the model with 3 example tenants and end-to-end customer flows.
The reference implementation for modern payments APIs. Used throughout M2-M3.