Not sure this is the place for these types of questions. But here's my 2 cents...
You still own the intellectual property rights to that code, if you can produce proof that you were the one who wrote it, then they will have to pay you. No contract has been signed, but if you have emails/recorded conversations of them agreeing to pay you in exchange for this work then that in itself is a binding contract.
They couldn't use your code in a commercial product without your written consent either, it's your intellectual property and they haven't held up their end of the agreement yet.
If it's a large sum of money it may be worth seeking legal advice, and don't ever think that you can't do anything about it. That's what people like that play on.
On the other hand, they may well be planning to pay and have just forgotten. You should try chasing them one last time and say that without payment you can't sign over the intellectual rights.
Maybe someone can correct me, but I'm pretty sure you're in the position of power here.