Does the colleague's repayment of the interest on the usurious loan, taken to purchase a car, after the borrower retracted and requested to return the amount for repentance, incur additional sin upon the borrower? And what is obligatory to do in this case, and upon whom is the repayment of the interest obligatory?

It is impermissible to borrow with interest (riba) to purchase a car or a dwelling, except in cases of dire necessity. The borrower must repent and promptly repay the loan if capable, and the interest should be waived. As for the colleague's disposal of the money, it constitutes an aggression and usurpation, necessitating the return of the money and any resulting fines. If the usurious interest is established merely by the act of borrowing, then the borrower bears it. However, if it is established due to a delay in repayment caused by the colleague, then the colleague bears it because he is the cause of it.