Is it permissible for the installment-payer to pay the cost of finishing the borrower's apartment in exchange for him paying the amount in installments?

The question's premise suggests that the questioner will pay money to workers to finish the apartment, and then receive more than what they paid, deferred, from the owner in installments. This is explicit usury (riba), as it is money in exchange for a larger amount of money guaranteed until a future date.

The transaction can be corrected if the questioner themselves undertakes the work of finishing the apartment for a known amount payable at a known time. In this scenario, they would be the contractor, bringing in workers and materials and overseeing the execution, or they could contract independently with another contractor. This method resembles installment selling, which is permissible after the seller takes possession of the commodity and it enters their liability. This differs from paying the price of a commodity in advance on behalf of the buyer and then receiving more than what was paid, deferred, as this is also usury.