For an initial implementation, I would use a modular monolith, PostgreSQL, object storage, a durable job queue, and integration workers. I would not start with microservices, a dedicated graph database, or a complex streaming platform.
The difficult scaling problem is likely to be heterogeneous financial data and exceptions—not raw request throughput.
A. Preserve immutable source evidence
Retain the original API payloads and uploaded files, with tenant, source account, retrieval time, source identifiers, and content hashes.
A reviewer should be able to trace a computed amount back to the original record. Re-imports should not silently change historical conclusions.
B. Model the financial chain explicitly
Create separate entities for invoices, payments, balance events, FX conversions, fees, payouts, bank credits, remittance documents, and declarations.
Connect them using value allocations, not just foreign keys or date similarity. One payment can cover several invoices; one invoice can have several payments; one payout can contain many payments, refunds, and adjustments.
Every allocation should record its amount, currency, evidence, matching method, confidence, and approval state.
C. Use deterministic reconciliation before AI
Start with exact transaction IDs, payout IDs, invoice references, bank references, and validated component totals.
Use controlled rules for known transformations. Use fuzzy matching only to propose candidates.
A useful stress test is a merchant with hundreds of identical $99 subscription payments. Amount and date alone are not enough to establish which invoice a remittance covers.
D. Maintain an auditable reconciliation subledger
A multi-currency subledger should track processor balances, receivables, fees, payouts in transit, refunds, reserves, and FX effects. Correct errors through traceable adjustments or reversals rather than overwriting history.
This should support the customer’s accounting system, not silently replace its booked revenue or exchange-rate policies.
Build invariants such as opening balance plus movements equaling closing balance, no double allocation, and payouts tying to their component records and bank receipts.
E. Treat reliability as a product feature
Use account-scoped idempotency keys, signed webhooks where supported, polling/backfills, retry queues, pagination checks, and explicit data-completeness indicators.
Handle late refunds, cross-month payouts, delayed documents, and historical corrections deliberately. A green reconciliation result based on an incomplete import is worse than a visible failure.
F. Use AI in bounded roles
AI can help extract document fields, classify descriptions, suggest mappings, and draft an exception explanation or bank email.
It should not invent missing amounts, certify a tax position, produce a fake remittance certificate, or approve an ambiguous allocation.
The valuable system is deterministic financial controls with assisted exception handling, not an LLM making accounting decisions.
Security and the regulatory boundary
Use least-privilege authorization, encrypted credentials, tenant isolation, role-based access, reviewer approvals, audit logs, tested backups, and explicit retention policies. Avoid collecting cardholder data you do not need.
Keep the initial product read-only and non-custodial. RBI’s 2025 Payment Aggregator Directions distinguish aggregation and settlement of customer funds from technology-only activities, and impose substantial authorization/capital requirements on payment aggregators. They specify ₹15 crore net worth at application and ₹25 crore by the end of the third financial year after authorization. Reserve Bank of India
That is a strong reason not to drift casually into collecting or settling money. Obtain a written legal-perimeter assessment; “we are SaaS” is not, by itself, a regulatory analysis.