You first inform the dealership owner of this salesperson and make him or her understand that any errors in a contract for a cash sale were the dealer's fault and not yours. By the dealership accepting full cash price for said vehicle, it accepted it as full satisfaction for the sale of the vehicle. As to additional efforts, immediately contact the city attorney or county attorney and file a complaint. If this dealership is regulated by any agency in your state, file a complaint with the agency (probably DMV).