What is the ruling on taking a cash sum equal to the value of a car, then repaying it in installments with an increase of five thousand Riyals?

The stated condition is void, and it is not permissible to borrow money with a condition of an increase on the borrowed amount, because this is precisely usury (riba). The jurists have unanimously agreed that stipulating an increase for the lender invalidates the loan contract and is considered usury. The alternative is for him to buy the car for himself and take possession of it, then you buy it from him at the agreed-upon price.