Like others have said, talk to a lawyer. But first - who owns the source code? Are there legal protections in your country (assuming not US) like a copyright

Like others have said, talk to a lawyer. But first - who owns the source code? Are there legal protections in your country (assuming not US) like a copyright? That could be your leverage in a business negotiation if you own the code. If your partner doesn't want to entertain renegotiating, then maybe say from this point forward (since he is aiming to sell the business) that any dev work needs to paid for separately from the royalties on existing work. Find other work in the meantime (maybe before this step).