This is more than just a legal issue though...
this is a moral one...
The Dealer made a mistake. What if you don't have that money now, because you assumed the first amount you paid was right, and now you don't have the money???
I'm in the same situation kinda, but the dealer doesn't know it.
I had the dealer install a Rhino lining, and they wrote down on the bill, that it was paid for as part of the sales deal.
Well, I knew that wasn't true, but it was there mistake.
4 months later, they still haven't called, and I bet they never will....but if they do, its a big F*** O** as far as I'm concerned...
It's THERE DUTY to provide a accurate bill/receipt for you. If you have a receipt/paperwork that says you paid for everything, then as far as they should be concerned, you are done.
I think a small claims court might agree, but it could go either way.
Stricktly by the law, the court might make you pay, but they may say because the dealer had a duty to ensure the paperwork was 100% accurate, that any mistakes on their part that benefit you, you can keep.
Small claims courts sometimes don't always follow the letter of the law, so you never know.