Is it permissible to conclude a promise to sell five cars by installment, to be received immediately, in exchange for a sum lent to him, and a monthly installment until the full price is paid, knowing that the cars are not currently owned by the seller but are by way of leasing? And does this fall under selling what one does not possess? If it is not permissible, what is the legitimate way to acquire these cars from him?

It is permissible for the tenant to rent out the leased property for the same rent, or for more or less. As for the landlord’s promise to sell you the leased property once it comes into his possession, there is no harm in it. This is not a sales contract, and it is not permissible to enter into a lease-to-own contract for it because he does not yet own it. He may only lease it to you, and he may promise to sell it to you once he owns it without entering into a binding contract to that effect; otherwise, it would be considered selling what one does not own. If the necessity to lease at less than the market rate is in exchange for delayed payment, then it is forbidden because it is usury.