Repaying debt is just giving back something you borrowed from someone.
Jesus taught his followers that if someone asks to borrow something then give it to them. He also taught that if an enemy wants something from you then give it to them and don't ask for it back. Matthew 5, Luke 6.
I suspect if you are dealing with a bank then you may not be dealing with Christians, but the bottom line is if you owe them something, they require it back, and you're in a position to do it (i.e., you have an income) then it's stealing to not return it and the secular law sees it the same way as well.