Is it permissible for a merchant, when purchasing a car in installments, to write the remaining amount as promissory notes (provided that each note is for the value of the monthly installment, and upon payment of the installment, he gives me a receipt for it) in order to avoid a usurious loan?

It is permissible to purchase a car by installments at a price higher than its cash sale price, provided that the term is known and the contract is free of prohibited conditions such as late payment penalties or the seller retaining ownership. It is also permissible for the seller to take guarantees such as a mortgage, a guarantor, or checks for the installments. As for the "amanah" receipt (trust receipt), it is not recommended to use it unless no other guarantee is possible, due to its potential for falsehood and invalidation, though it is permissible in this case for the sake of benefit.