Is it permissible to use surplus money to pay off part of a usurious loan, while taking interest from the bank to pay off the remaining installments? And does the noble verse apply to this situation: "And if you repent, then you shall have your principal sums"?

It is not permissible to deposit money in a riba-based (usurious) bank to pay off usurious loans from its interest. However, if you are compelled to take out a loan with riba, there is no sin upon you. It is not permissible to benefit from usurious interest to pay off debt or for any other purpose; rather, you must dispose of it by giving it to the poor and needy. If accelerating the repayment of usurious debts waives the interest, then you must hasten to do so. You are not obligated to pay the interest on a usurious loan, but only its principal. And the interest from a usurious deposit is not permissible to benefit from.