Backend Engineer (Payments) — Savannah Pay

About the role

This role is at the technical core of Savannah Pay: building the payment processing microservice that handles every send, receive, and convert operation across five African markets. You will design an idempotent transaction system that handles the inherent unreliability of mobile money APIs — where callbacks are delayed, duplicated, or occasionally never arrive — without ever double-charging or losing a customer's money. Your primary responsibilities: implement the Safaricom Daraja API integration for STK Push, B2C, and C2B flows; build the currency conversion service that sources rates from a central bank feed and adds a transparent margin; design the database schema for ledger accounting using double-entry principles; build the reconciliation job that compares our internal ledger against M-Pesa transaction statements; implement the idempotency key system that prevents duplicate transactions on retry; and build the compliance reporting module that generates CBK and FCA regulatory reports. You will also build the webhook delivery system that notifies our Flutter app and partner systems of transaction state changes, with retry logic, signature verification, and dead-letter queuing for failed deliveries. The technology stack is Node.js 20, TypeScript, Fastify, PostgreSQL with row-level locking for transaction safety, Redis for idempotency key storage, and Kafka for event streaming. You will deploy on Kubernetes on AWS EKS and use GitHub Actions for CI/CD. Ideal candidates have experience building payment systems, understand the Daraja API deeply, and have worked with event-driven architectures. Experience with reconciliation systems or double-entry accounting in software is highly valued. The role includes a performance bonus tied to zero payment errors at launch.