Did you ask him for the balance of the money back? Maybe he was assuming that he could continue paying it off at $50 a month regardless - sounds pretty selfish to me, but anyway. But by stiffing you (and others too?) he's going well on the splitting-up front himself.

Make damn sure he's no intention of giving you your money back though, if you do sue. Suing's very much a last-ditch thing, only to be tried after all polite (and impolite shouting-and-screaming) methods have failed. The only person who really wins a court case is the lawyer. And then cuz offers $10 a month and the court says "fine", and you're worse off than b4, plus court fees...

Grab.

PS. Good luck - stuff like this is never pleasant.