What is the ruling on the method of purchasing homes from elderly non-Muslims, where a portion of the price is paid upfront and the remainder in monthly installments, concluding either with the full payment of the amount or the death of the seller, with varying conditions for occupancy?

If the price of the house is agreed upon to be divided into installments, the sale is valid, whether or not an upfront payment is made. However, if the price is not agreed upon, and it is merely monthly installments that cease upon the death of the owner, the contract is invalid due to the uncertainty of the price. In the case of a permissible sale, the seller may stipulate living in the house for a specified period; if the period is not specified, it is not permissible.