What is the ruling on taking a loan from an interest-based bank and not investing the borrowed money, and is it sufficient to repay the loan?

This person must sincerely repent to Allah and frequently seek forgiveness and perform good deeds. Dealing with usury is one of the major sins for which Allah has declared war on those who engage in it. The Messenger of Allah, peace and blessings be upon him, cursed the one who consumes usury, the one who gives it, the one who records it, and its two witnesses.

If the person has fully repaid the loan to the bank, then the money in their possession belongs to them. If they have not repaid the entire loan, then they should try to reach an agreement with the bank to waive the interest and pay the remaining amount promptly. If they are unable to do so, then they should pay according to their ability and always turn to Allah in repentance.