This is a weird one, but I'd say (not having any legal basis, just personal opinion) just leave it alone and let the client do whatever, unless it's some real large chunk of change you're talking about. Then you MAY be able to fight it in court, but I'd consult with a lawyer first.

If the client is cooperative, get a contract now and strike up some kind of fair deal. If not, count your losses and move on with a learning experience.