Basically, you demand payment and you demand that all communications be done in writing. And he calls you, you politely hang up the call. "I'm sorry. I can't talk right now. Please communicate to me in writing." Whatever you do, do not give the source code now, that's your only leverage. If your employer wants to say that you had a verbal agreement, let him say this to you in writing. The more he admits in writing, the more you'll be able to prove you had some kind of agreement with him.