Is buying an apartment from a housing association that requires payment in installments, knowing that the association borrows from the bank to complete construction, considered unlawful (haram) or usurious (riba)? What is the correct action to take if it is deemed unlawful? Is it permissible to sell the reservation in this case?

The association's borrowing from a usurious bank with interest is forbidden by Islamic law (haram), and the sin falls upon those in charge of it. Those dealing with the association in building apartments are not held accountable unless the association is acting as their agent. It appears from the question that the contract between the questioner and the association is invalid, due to the ambiguity in the price between cash and deferred payment, which falls under the prohibited category of "two sales in one sale." The contract must be rescinded and then re-executed based on a specific, determined matter. It is not permissible to sell the apartment to the questioner until he takes ownership of it by rectifying the contract and receiving it after the completion of construction.