PennyWise

PennyWise is the backend for a savings and payments app. Each user gets a virtual account and a wallet, plus the plumbing to move money between them: fund by bank transfer, send to another user, or withdraw to a saved account. Payments run through Flutterwave. Withdrawals are gated behind an OTP and a transaction PIN, and a webhook reconciles every transfer against its stored reference.

I'd been using a savings app and kept wondering how the money side actually worked, particularly virtual accounts, settlements, and what happens when a transfer half-completes. Reading about it wasn't enough, so I built the backend myself.

The hard part isn't the CRUD. It's that the payment provider, not your database, is the source of truth for money. A withdrawal can succeed at Flutterwave and never reach your app; a deposit can land before you're ready for it. So the system is built around reconciliation: every transfer stores a reference, and a webhook settles it after the fact rather than trusting the request that started it.