There are often such problems with contract work. My attorney explained to me that if the developer does not sign over the rights of the code developed it could be subject to license fees. If you seek an attorney for this they will say something like pay this invoice or pay a license fee for the code. Once they pay, you will need to sign over all rights to the code. There are attorneys that work with developers on these type of subjects as everyone I know has issues on both sides of contract work.