IANAL...
Good news: Everything is renegotiable! New contract, new negotiation. This is a two-way street.
Don't let them manipulate you into signing a new contract without concessions on the price.
If your wife has been driving that car for two months, you may have them over a barrel. If she returns the car, it is no longer new. That can take a lot off the value of the car (10-20%?). Because these are sleazy guys, I will assume they would try to sell the vehicle as a dealership demo model (they can't erase the mileage), that is still worth ~$500-$1500 less than a spanking new car.
I would guess they are looking at a $1000 hit in the pocketbook if you end up returning the car after driving it around for two months. (Maybe more, if you include the hassle factor.) The extra 4 months of payments are, say, $2000?
As you can see, they have clear incentive to try and bully you into eating the costs of their mistake. If you really want the car, there may be room for concessions on both sides...
Of course, if I were you, I would be sufficiently teed off that I would just walk away from the deal. There are other dealerships out there waiting for their chance to screw me over.
