回复：回复：怎么办？

Your advice is profession. But it's either too time-consuming or money-consuming. Since the buyer seems to want the car, the best way is to simply tell the manager (or manager's manager) that a complaine will be made (to attoney general's office or/and better business bureau) and that the buyer will take any future business elsewhere. A dealership probably makes more money maintaining and repairing a car than selling one.

Ask the dealership whether they are willing to lose the business for a couple points of interest rate, which they don't really benefit that much. They will come to their sense, hopefully.