copyright ownership of work for hire

[DickieBow]

I was commissioned to develop a software game as a work for hire. Although I've assigned copyright in the work to the commisioner of the work, the commissioner has not paid for the work in full (about 25% of the flat fee is outstanding). The commissioner is not responding to requests for payment, and seems to be deliberately ignoring me. The software is due to be published in the US and worldwide shortly. What, if any, legal remedy do I have to this situation? And can I revoke copyright ownerhip of the game, given that I have not been paid the agreed monetary fee?

I live in England.

Thanks

 

[racer72]

U.S. law only.