"no debt remains unpaid" means that even though how much debt you have on that someone, you can always pay him with something that he can't even just thank you. for example, you have millions of debt on that someone because you lose on some games in casino, and he always ask you to pay him, but what if sometime you saved him even risking your own life? would he still ask you to pay him? the right thing should be that he should be the one who have to pay because not because of you he can live longer in this world.
