The Gemara [Ksubos 86a] says that acc. to the מאן דאמר that שיעבודא לאו דאורייתא, the borrower is forced to pay back the loan b/c פריעת בעל חוב מצוה.
If so, he should only have to pay if the loan has the value of up to one fifth of his total possessions b/c מצוות are עד חומש??!!! [And we know that this isn't so!!!?]
