Is there sin for one who pays the installments on a social housing apartment belonging to his absent brother-in-law?

Buying apartments from the Social Housing Bank takes two forms:

1. Direct purchase from the Urban Communities Authority in installments: This is permissible and presents no issue. The bank's role is limited to paying the installments through its channels.

2. The bank acting as a financier:

Permissible Murabaha: If the bank first buys the apartment and then sells it to the client at a profit, provided the contract is free from prohibited conditions such as late payment penalties, suspension of ownership until payment, or prohibited insurance.

Prohibited Usurious Loan: If the bank's role is solely financing, meaning it pays money on behalf of the client and reclaims it with a stipulated increase. In this case:

If the apartment has not been specified or a contract with the bank has not been made, the transaction must be stopped and the money withdrawn.

If the contract has been made and the apartment specified, then the contract is a prohibited usurious one, requiring repentance. There is no harm in paying the installments and benefiting from the apartment, along with repenting from usury, if the contract cannot be rescinded without excessive harm. There is no harm for others to pay the installments on one's behalf.