I had this same deal happen with me when I bought my Durango R-T in 2000
Went to the dealer with my 97 Thunderbird LX to trade in on the Durango. The salesrep did all the work ( running the numbers, getting the paperwork together, how much my trade in owed, would sell for, etc )
30 days later I am driving to work in my new truck and I get a call that they were $2500.00 short and needed the money and the truck returned to the dealer.
I told them that if they can produce my car that I traded in to them in the same condition that I gave it to them in then I'd consider coming down that weekend.
The dealer said he'd have to talk to his boss and get back to me.
July 22nd - It'll be 6 years since he was to get back to me
I'd tell them to get fucked.