eu31, first of all, NOT the bank owes him. It's the other way around: He is the one who owes the bank.
second of all there are different types of the loans: with the fixed interest rate and with the non-fixed interest. Probably the guy got the second type of the loan, and might not be aware of it.

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“You become responsible forever for what you have tamed”. Antoine de Saint-Exupéry