Senior Developer for a Fintech SaaS

We’re looking for a Senior Full-Stack Engineer with hands-on experience building financial software for the Spanish market. This is not a typical CRUD invoicing application. We’re building a modern ERP/Business Management platform already used by real customers, and we’re looking for someone who understands both the technical architecture and the business, accounting and legal requirements behind financial software. We’re looking for someone who has worked on products similar to Holded, Sage, Odoo, A3, Quipu, Anfix or other accounting/ERP platforms, and who understands how these systems are expected to behave. What you’ll build You’ll work on a complete financial workflow covering the entire business lifecycle, including: * Quotations with version history and approval flow * Conversion of quotations into projects/orders and invoices * Customer invoices * Supplier bills * Proforma invoices * Recurring invoicing * Credit notes (corrective invoices) compliant with Spanish regulations * Fiscal numbering sequences * Multiple numbering series * Payment tracking and due dates * VAT, withholding taxes (IRPF) and different tax scenarios * Multi-company organizations (holding structures) * Complete audit trail and document traceability * Foundations for Verifactu, e-Invoicing and future Spanish regulatory requirements Every financial document must always be linked to the correct legal entity, fiscal series and accounting context. Tech Stack * TypeScript * Next.js 16 * React 19 * PostgreSQL * Drizzle ORM * Redis Required experience You have real experience building financial software such as: * ERP systems * Accounting platforms * Invoicing software * Fintech products * Business management software You should already be comfortable with concepts such as: * Quote → Order/Project → Invoice workflows * Credit notes * Fiscal document numbering * VAT * Spanish withholding taxes (IRPF) * Tax calculations * Due dates * Payment status * Accounting traceability * Multi-company environments * Financial reporting We’re not looking for someone who wants to learn these concepts during the project. Technical requirements * Advanced TypeScript * Strong React / Next.js experience * Advanced PostgreSQL * Excellent SQL skills (transactions, locking, indexing, referential integrity) * Safe database migrations on production systems * Designing complex relational data models * Correct monetary calculations (no floating-point arithmetic) * Decimal precision and tax rounding * Idempotent APIs * High code quality and strong attention to detail Nice to have Experience implementing Spanish financial regulations, including: * Verifactu * Electronic Invoicing * Spanish Anti-Fraud Law * TicketBAI * SII * Spanish invoicing regulations * Fiscal numbering requirements * Legal document immutability Also highly valued: * Multi-tenant SaaS architectures * PostgreSQL Row Level Security (RLS) * Audit logs * Event sourcing * Hash chains * Financial-grade security and data integrity Who we’re looking for We’re looking for a senior engineer who can think beyond implementation. You should be able to identify legal, accounting and architectural implications before writing code, propose robust solutions, and help design a financial system that is technically solid and compliant with Spanish regulations. This role is ideal for someone who has already built serious financial software and understands what users expect from modern platforms like Holded, Sage or Odoo—not just from a coding perspective, but from an accounting and regulatory one as well. Thanks for your time, and hopefully we’ll talk soon.