Is the aforementioned transaction for purchasing a house through partnership with a real estate office, where the office pays 80% of the house's value and the buyer pays 20%, and then the office sells its share to the buyer in installments with a fixed profit margin on monthly payments for a specified period (up to 25 years) without changing the installments, considered permissible and lawful in Islamic law?

There is no harm in partnering with a real estate company to purchase a property, with the company promising to sell its share to you in installments, either in a lump sum or gradually (diminishing partnership), provided that the contract does not include a penalty clause for delay. If you purchase the company's share, ownership transfers to you immediately upon contract, even if the installments extend over many years. It is permissible for the company to sell its share in annual installments, whereby your ownership gradually increases. The Islamic Fiqh Academy has ruled the permissibility of diminishing partnership, provided that the selling price is determined at market value on the day of sale or by agreement at the time of sale.