Is purchasing a housing unit from a cooperative housing project that relies on usurious loans considered usury? And is the contract for purchasing the apartment from its first owner, which stipulates that ownership does not transfer until the full price is paid, Islamically invalid? And is it permissible to sell this apartment in an Islamically permissible manner, and what is the advice for someone who purchased this apartment without sufficient Islamic knowledge?

It is permissible to purchase apartments before they are built or before their construction is completed, provided the purchase is based on known specifications that eliminate ambiguity. This is called a "contract of istisna‘" (manufacturing contract), and it requires knowledge of the specifications of the subject of the contract and the determination of the delivery time.

It is not permissible to deal with banks or institutions that pay the full price or part of it to the seller, and then the buyer repays it to the bank with an increase, as this constitutes usury (riba).

It is permissible for you to buy what someone else has purchased through usurious banks, and it is better to buy from them after their relationship with the bank has ended.

If the apartment is mortgaged until its full price is paid, then selling and buying it is not valid and involves risk, because the bank can seize it in case of delay in paying the installments.