What is the ruling on real estate financing when there is a tripartite contract, and is it permissible to deposit the money in the remainder to benefit from the installment value?

The questioner did not mention anything about the apartment purchase contract and the method of real estate financing to determine the validity of the contract. If the bank is Islamic and the financing is through legitimate Murabaha (the bank first bought the apartment then sold it to the questioner at a higher deferred price), then there is no harm. However, if the bank is usurious and the financing is a usurious loan, then the contract is forbidden, and the questioner must repent. The apartment is his property, and there is no harm in living in it, because the sin of usury is not attached to the borrowed principal itself.