What is the ruling of Islamic law regarding a person handing over his car to one of his friends for the purpose of selling it, then this friend sells it and seizes its price, refusing to hand it over to the car's owner?

If the agent seized the car's price and denied it, he is usurping the owner's money, and this is forbidden. The principal can sue the agent to recover his right, or secretly recover the money if possible. He may also expose him, based on the verse: "Allah does not like the public mention of evil speech except by one who has been wronged."