My advice may be late, but it may still help others. I had a similar situation about a year ago when I bought a new car. Agreed on 2.9% financing, signed a contract, and drove away with my car thinking to myself, "wow, that was just to easy."
Well it was. About 10 days after I bought the car, the succubus (finance manager) calls me to inform me that the 2.9% rate was not valid on the day I bought the car. It had apparently be cancelled retroactively! She wanted me to return the next day and sign a new contract for 5.9%!
I told her that this was the most ridiculous thing I had ever heard, and that I would not be returning, and that she should call me back when she works out HER financing dilemma. What racer72 told you is absolutely correct. Sign new papers, or return the car. But there is a third option... do not give in, make the biggest scene and cause them as much grief as you can.
I discovered that this is what's called spot-delivery-scam. They give you a contract, which in reality they don't have to honor until they assign financing, and tell you to drive off with your car. Once you do, technically you are stuck. But it's up to them the amount of effort they put forth to "assign" the financing.
The Finanace Manager knew in advance she should not have given me this rate. I know this because she was stupid enough to fax me an internal memo from their vice-president of marketing cancelling this special rate... it was dated two days before she sold me the car. Nevertheless she knew I wouldn't buy the car but for this interest rate. Due to her own greed, she figured, "ah what the hell, tell the guy it's this 2.9% rate, then screw him over later."
Anyway, to make a long story short (yeah I know, too late) after bantering back and forth with the succubus for about a week, I sent the manager of the entire dealership group a letter informing him of the situation. I also told him that if they did not honor the contract within 7 days I would take my issue to the FTC, Attorney General, BBB, and the media.
Much to my surprise I got a call at 7pm on day seven. Apologies all around! "Of course we will honor the contract, please forgive this misunderstanding, we'll eat the difference." What happened in the end is this: I got the car at 2.9%, the dealership had to finance it at some higher rate, but they are the ones who had to pay the difference.
Hope to God this helps someone else out there...