What is the ruling on selling a car in installments at a higher price than its cash price, then the seller's father pays the amount to the seller, on condition that the buyer repays the amount to the seller's father in easy installments without a precise due date?

There are two contracts: The first is a sale between the questioner and the car owner, and the seller's father will pay the price. Installment sales are permissible, subject to their conditions.

The second is a loan between the questioner and the seller's father, which is unrelated to the sale contract. The condition of a term (for repayment) in a loan has been previously explained.