The ruling on the contract of istisna', such as a contract to purchase a house to be built in the future

Question:
He says: Honorable Shaykh, may Allah grant you success. Some companies build a house for the one who contracts with them, on the condition that he pays the price in installments over one or two years. At the time of the contract, the house does not exist, but it is described by its characteristics so that the buyer is fully informed. As soon as the contract is made, the company begins building the house. What is the ruling on this?

Answer:
This is not valid, because it is the sale of something that does not exist. This is what they call istisna'. This is not permissible because it is the sale of something that does not exist; it is the sale of something that does not exist. However, if the house is completed and finished, you may buy it after it is completed and finished. If the company wants to do what is correct, it should build the houses and offer them for sale, so that whoever wants to buy can purchase from them.