ואם לוה ככר קודם פסח, מוטב להחזירו קודם פסח ואם לא החזירו קודם יחזירנו אחר פסח ואין בזה חשש, ואם לא יחזירו יש בזה משום גזל: - if one borrows a loaf of bread before Pesah, it is better to return the loaf before Pesah, but if one didn't one should [definitely] return it after Pesah and there is no need to worry. And if someone did not return the loaf after Pesah, it becomes a problem of robbery.The Ben Ish Hai (Year one, parashath Tzav, letter yud) explains an interesting dilemma: If you owe someone a loaf of bread before Pesah, and you give them back a loaf of bread after Pesah, is this problematic?
It is better to return any debts of Hametz before Pesah. Though, if one did pay back the debt after Pesah, there is no problem with it. (For those who would like to look into this halachah in further depth, it is discussed in Siman תנ - 450 in the Shulchan Aruch, Beit Yosef, Aruch HaShulchan etc. The explanation in the Shulchan Aruch HaRav (found online here) is especially clear and thorough.)
To me this is an interesting question. It's as if we're getting into the realm of quantum-mechanics-like questions. Does the debt purport some kind of potential existence of this loaf of bread over Pesah? All the halachic inquiries deal with whether there was any kind of connection between the debt and any actual hametz that might be said to be in quasi-possession of the Jew over Pesah. In the end, even if a non-jew pays back a debt to a Jew (from before or (i think even on Pesah)) after Pesah with hametz that was prepared on Pesah, there is still no problem.
When I learned this halachah I liked the intellectual elegance of it; But it took a practical turn when I remembered a coworker bought me a KitKat (actually the Israeli equivalent: KifKef) a month or two ago and I never got around to paying them back. Now I know I should probably do it before Pesah to avoid any doubts.