Is the contract with the bank to complete the purchase of an apartment for low-income individuals—which includes conditions such as the inability to pay in a lump sum, the requirement of a general power of attorney for the bank over the apartment, taking checks for the amount, and granting the bank the right to dispose of the apartment in case of non-payment—considered a sinful contract?

The problem in this transaction is the signing of the contract between the bank (financier), the seller (owner of the apartment), and the buyer, where the bank does not own the apartment but pays its price immediately to the seller, and then takes it later from the buyer with an increase. This contract leads to usury (riba). This is unlike a situation where the bank first buys the apartment and then sells it to the buyer, which would constitute a Murabaha sale to a party requesting the purchase. Since the questioner entered into the contract unaware of its true nature, and it has already been executed, and he cannot retrieve what he has paid, he should complete the transaction because the forbidden act has already occurred. He should not leave his money to the usurious bank. He should seek Allah's forgiveness and avoid similar situations in the future.