Is a real estate sale valid in Islamic law when most of the price has been paid in cash and the remainder in installments, provided that the seller stipulates not to transfer ownership of the title deed to the buyer until all installments have been paid, or is it similar to a lease ending in ownership?

The transaction mentioned is a sale, not a lease ending in ownership transfer. If the failure to transfer the title deed of the building to the buyer means that ownership is not transferred to him until all installments are paid, such that ownership remains with the seller during that period, then this is a void condition, as it contradicts the essence of the contract. However, if the intention is to mortgage the building to prevent the buyer from disposing of it until the full price is paid, then this is a permissible condition. The Islamic Fiqh Academy has permitted mortgaging the sold item for its price while prohibiting the seller from retaining its ownership.