Without any explicit IP transfer agreement, you likely own the code you produced (even if they paid you) and could use that as leverage to resolve the dispute.

Without any explicit IP transfer agreement, you likely own the code you produced (even if they paid you) and could use that as leverage to resolve the dispute. Nonpayment sometimes happens with freelance work -- it may not be worth the stress getting payment for the final invoice. Because they threatened to sue, for peace of mind you could also find a lawyer and ask them to send all future communication to the lawyer.