C.P.
Did you guys get gap insurance (something like that) that covers the difference in cases like this? We had gotten it with one of our cars after having that happen to us before and we lost out. Maybe you got it and didn't know about it.
But to answer your question, I don't know. I think nowadays they may be willing to do anything to get you to drive a car off of the lot (and don't forget to get gap coverage or ask about it just in case) so they may be willing to roll the balance over into a new loan for another car.