Senior Java/Spring Developer – Fintech & AI Platform (Payment Integration)

We're building a fintech platform powered by AI, and are looking for a senior Java developer to join a part-time, well-scoped project involving payment gateway integration and backend bug fixes. Scope of work: Integrate international payment gateways, including Stripe, Google IAP, and Apple IAP Ensure transactions across all gateways are processed correctly, securely, and reliably Resolve existing backend bugs within a complex, production-grade codebase Improve overall platform reliability and stability Work comfortably with legacy code, identify root causes of issues, and deliver stable, well-tested fixes What we're looking for: Proven experience integrating payment systems (Stripe and/or mobile in-app purchase systems a strong plus) Comfortable navigating and debugging complex, existing backend systems Strong communication skills — this is a part-time engagement, so clear async updates matter Ability to work independently while delivering reliable, production-ready results This is a part-time role with a clearly defined scope, on a fintech + AI product with real technical complexity — a good fit for someone who enjoys solving non-trivial backend and integration challenges.