Full-stack developer — multi-currency treasury / money-services platform

We are building an internal operations platform for a money-services business. It includes a multi-currency double-entry ledger, client / partner / aggregator / sub-agent accounts, payment workflows with verify and approve controls, FX handling, and a full audit trail. Detailed functional specification, UI/UX specification, data model, and a working UI prototype are already prepared. Must have: PostgreSQL; strong backend (Node.js or Python); prior experience building accounting / ledger or fintech systems with transactional integrity. Nice to have: multi-currency / FX, role-based access control, React, WhatsApp Business API. To apply: briefly describe a double-entry or financial system you have built and how you ensured data integrity. Fixed-scope first phase with clear written specifications provided.