Is it permissible, Islamically, to resort to a financing company accredited by the bank to provide a guarantee in exchange for a 2.5% commission, in order to cover a down payment for purchasing an apartment through the bank, knowing that the payment will be made directly to the bank and the apartment will remain in the bank's name until the full amount is paid?

A financing company taking a commission for a guarantee is impermissible because a guarantee is a contract of aid (irfaq), not exchange (iwad), and it is not permissible to guarantee for a fee.

For the bank's purchase of the apartment to be permissible, the bank must first acquire ownership of it, and then sell it to the customer through a murabaha sale to a party promising to purchase. In this case, the bank is permitted to prevent the customer from disposing of the apartment by selling it until the full price has been paid.