Is the borrower required to compensate the heirs for the value of the two cars from his own money after their destruction while in his custody?

If the son used his father's car after his death without the permission of the heirs, he is absolutely liable for its damage. However, if it was with their permission, he is entrusted with it and is only liable in cases of negligence. As for the other car, it is not permissible to leave it at the workshop without a fee, because he is an agent for the heirs and must act in their best interest. He is obliged to pay the value of the burned car if his use was without permission or if he was negligent with it, and he must sell the other car as requested by the heirs or pay its value if he left it at the workshop without a fee, unless the heirs waive their right.