What is the ruling on a father selling his car to a merchant on credit, and then his son buying it cash from the merchant in the same sitting for a lower price?

If your father sold the car to the merchant on credit, and the merchant took possession of it, then you bought it from him for cash for yourself, and you were not acting as an agent for your father in the purchase, then the transaction is permissible. This is based on the statement of Ibn Qudamah: "It is permissible for others—whether his father, his son, or anyone else—because he is not the seller and he is buying for himself; thus, he is like a stranger." However, if the intention was to use this as a trick to lend the merchant thirty thousand to be returned as fifty, then this is forbidden, based on the statement of Al-Mardawi: "If it was a trick, it is not permissible."