Decide who writes a billing engine's money math

Your product manager wants an AI agent to build the core of a new billing engine in Python. That core covers proration when customers change plans mid-cycle, tax, rounding to each currency's smallest unit, and credit notes. Invoices go to customers in 30 countries, and finance closes the books from them every month.

Which parts, if any, should the agent build, and which do you keep? Defend the split.