Am I obligated to return a sum of money to the buyer, knowing that I was unaware of the true cause (the cooling system) of the recurring gearbox defect before the sale, and the buyer also inspected the car and did not discover that defect?

If someone sells a car with an old defect that the seller was unaware of, the buyer has the option to either annul the sale or accept it while receiving compensation for the defect, unless the seller stipulated exemption from all defects of which he was unaware. If the buyer repairs the defect, there are two views: The first view holds that the compensation (arsh) is forfeited by the buyer's action, while the second view holds that the compensation is not forfeited, and the seller is obligated to pay the repair cost.