Mishnah 3 in the second chapter of Maseches Pesachim deals with the status of chametz that serves as collateral for a loan - whether the lender is a non-Jew or a Jew - and also with the status of chametz buried beneath a collapsed building.
"Nochri shehilvah es Yisrael al chametzo" - a non-Jew who lent money to a Jew against his chametz:
A non-Jew lends money to a Jew and demands collateral for the loan, and the collateral he takes from the Jew is chametz - for example, a case of whiskey that the non-Jew took and brought into his own domain and into his house. The Mishnah rules that "achar haPesach mutar b'hana'ah" - after Pesach it is permitted to derive benefit from it.
This requires explanation, since on the face of it the chametz still belongs to the Jew during Pesach. The explanation is that we are dealing with a case where the Jew stipulated with the non-Jew at the time of the transaction: "If I do not pay you by such and such a date, this chametz will be yours from now" - meaning from the moment of the loan itself, which took place before Pesach. Once the due date arrived and the Jew did not pay his debt, it turns out retroactively that the chametz belonged to the non-Jew already before Pesach began, and it was never in a Jew's possession during the festival, and therefore it is permitted to derive benefit from it.
The dispute among the Rishonim regarding the due date:
The view of the Rambam: we are dealing with a case where the due date falls before Pesach.
The Raavad's question: if the due date arrived before Pesach, there is no need at all for the chametz to become the non-Jew's retroactively from the time of the loan, since the due date itself - at which point the chametz becomes the non-Jew's acquisition - falls before Pesach. The Raavad therefore establishes the Mishnah as speaking of a due date that falls after Pesach; and since they stipulated in the transaction that the chametz would be the non-Jew's retroactively, and the Jew did not repay the loan, it turns out retroactively that the chametz was the non-Jew's from before Pesach, and it is therefore permitted to derive benefit from it afterward, even though it originally belonged to a Jew.
"V'Yisrael shehilvah es hanochri al chametzo" - and a Jew who lent money to a non-Jew against his chametz:
In the reverse case, where a Jew lends money to a non-Jew and the non-Jew's chametz serves as collateral, and the Jew took the chametz into his own domain - "achar haPesach asur b'hana'ah" - after Pesach it is forbidden to derive benefit from it. Here too we are dealing with a scenario in which it was established in the transaction that if the non-Jew does not repay his loan, the Jew acquires ownership of the chametz from now, from the time of the loan, and therefore the chametz genuinely belongs to the Jew. For otherwise, this would simply be a non-Jew's chametz sitting in a Jew's domain, and in such a case it would be permitted to derive benefit from it. But here it turns out in the end that during Pesach the chametz belonged to a Jew.
"Chametz shenaflah alav mapoles" - chametz upon which a building collapsed:
Chametz upon which a building fell - "harei hu k'mevo'ar" - it is considered as though it has been destroyed. Even though we know that it is there, it is nevertheless buried to such an extent that there is no obligation to search for it. Even so, one must still nullify it in his heart, lest it be uncovered - for it is possible that some digging equipment will happen to expose it - and therefore we perform bittul.
Rabban Shimon ben Gamliel sets the criterion: "kol she'ein hakelev yachol l'chapes acharav" - anything that a dog cannot search out. A collapse is considered as such only when a dog cannot search out the chametz and dig it up, and the Gemara explains that this measure is a tefach. With less than a tefach we are concerned that it may be possible to extract the chametz, and that is a problem.
In summary: in this Mishnah we learned three rulings: a Jew's chametz given as collateral to a non-Jew is permitted for benefit after Pesach, because the non-Jew acquired it retroactively from the time of the loan (and the Rambam and the Raavad disagree as to whether the due date falls before Pesach or after it); a non-Jew's chametz given as collateral to a Jew is forbidden for benefit, since it turns out retroactively that it was in a Jew's possession over Pesach; and chametz upon which a building collapsed is considered as destroyed and there is no obligation to search for it, provided that he nullifies it in his heart, and the measure of the collapse is whatever a dog cannot search out, which is a tefach.