What is the ruling of Islamic law on reserving an apartment in a youth housing project by paying a down payment and the remainder in installments with a 5% interest rate, knowing there is a sum called "Owners' Association" that is deposited in the bank and its proceeds are spent on maintaining the building? What if the remaining amount is paid in a single lump sum? And is the "Owners' Association" sum considered usury (riba)?

It is not permissible to purchase the apartment in the manner described, due to the usury this entails. The remaining part of the price is considered a debt upon you, and if interest is calculated on it, it becomes a prohibited usurious debt. Even if the entire price were paid to avoid usurious interest, it would not be possible to avoid paying the homeowners' association fee, which is deposited in a bank and invested with usury.