What is the ruling on continuing to pay the commercial shop's rent to the lending merchant until the full loan is settled, knowing that the remaining loan amount is small?

If the paid rental amount reduces the debt equivalently, then there is no harm in it. However, if it does not reduce the installments correspondingly and is considered interest for delayed payment, then it is not permissible. This is because he is only entitled to his principal, and "But if you do not, then be informed of a war [against you] from Allah and His Messenger. But if you repent, you may have your principal – [thus] you do not wrong, nor are you wronged." Therefore, the rent payments should be deducted from the remaining installments, and only the remaining portion of the loan should be paid without any increase, as it is not permissible to obligate the debtor to an increase on the debt for his delay in payment, because that is forbidden usury.