Is the profit obtained from investing money in buying and selling cars by installment payment permissible (halal)? And what should be done if it is impermissible (haram)? And what is the correct Sharia-compliant action to make this transaction permissible?

If a person gives his money to someone to trade with it for a share of its profit, this transaction is called Mudarabah (commenda partnership), and it has its conditions and regulations. It is not permissible for the agent (Mudarib) to give the money to a second Mudarib unless explicitly or implicitly authorized by the owner of the capital.

If the owner of the capital gives permission, then the profit is divided between the owner of the capital and the agent according to their initial agreement. Then, the agent divides his share of the profit with the second Mudarib according to their agreement.

Therefore, what the questioner's relative did by giving the money to the car dealer is correct if it was with his permission, and the resulting profits are permissible. However, the car dealer guiding the buyer to the bank to borrow with usurious interest is forbidden and considered aiding in sin.

The questioner should prevent the agent of his money from engaging in such conduct in the future. The profits resulting from the transaction are permissible because the sale between the dealer and the buyer is valid, and the sin is associated with borrowing with usury and aiding therein.