The general rule is buyer beware. You cannot rescind a contract because you realize later that it was a bad deal. You can return the car if there was a return policy or contract provision that allows as much. Your dad also might have remedies if there was fraud on the part of the dealer, or some reason why he was unable to consent to the deal. You might want to consult with an attorney, but know there is a good chance your dad will be stuck with the deal.
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I would suggest contacting a local area consumer law attorney to fight this matter. You can find one or more in this very forum by going to the opening menu and typing in consumer lawyer and in the city/state, type in Rancho Cucamona, and California. How cognizant is your father at age 87? Best of luck.
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