This is tough and will be tough to prove, as you always want to have your agreements in writing, otherwise, the agreement is "as good as the paper it is written on." You can argue they violated the law by not putting the terms in your contract but it's not going to be easy. Maybe other people have been "taken" by this same tactic? If you find them, that should make your case easier to prove.
Find someone good here for a low or no cost consultation:
You are right and they are wrong, but proving it will be hard because it's your word against their's if it wasn't written down. If the amount is not too much, yes, you can sue the dealer in your local small claims court.
This answer is for general purposes only and does not establish an attorney-client relationship. The law in your state may differ and your best answer will always come from a local attorney that you meet with privately. For a Free Online 50 State National List of Consumer Law Lawyers, click on this link (http://tinyurl.com/79ku5jx) and find one near you