You need to pay whatever you agreed to pay at the time of the transaction--no more and no less. So if the purchase price of the new vehicle was, say, $5,000 plus your trade-in car, then you'd have to pay an additional $4,500. Once you have agreed to the transaction, the terms cannot be unilaterally changed after the fact; so look to the agreed-upon sale price.Note that if the bank is not a party to the transaction--e.g. it's not the seller--but is being looked to to provide financing, that's a different matter in this way: while the bank cannot increase the purchase price (amount due the seller) for the new car, the bank does NOT have to give you financing except on terms it finds acceptable. So say that the purchase price is $5,000 plus the trade in. If the bank deems you a bad risk, it may require you to have more "skin in the game"--i.e. to put more money down. So instead of paying $500 down and financing $4,500 in this case, the bank may say that you have to pay $1,500 down and only finance $3,500--which still totals to the original $5k. You may look to other lenders, either on your own or with the seller's help, to try to find a better deal.