This Halacha is an excerpt from our Sefer
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Chametz of another Jew:
It is forbidden for one to have Chametz that belongs to another Jew remain in one’s home on Pesach even if one has no liability over the Chametz, unless that Jew performed Mechiras Chametz. Accordingly, one who is storing the Chametz of another Jew in his home is to contact him prior to the start of the 6th hour on Erev Pesach and verify that he sold his Chametz. If one is unable to verify if the Jew sold his Chametz to a gentile, then he is obligated to sell the Chametz for him, prior to the 6th hour on Erev Pesach. If it is already past the 6th hour on Erev Pesach, and one does not know if the owner sold his Chametz and one did not do so for him, then one is obligated to destroy his friends Chametz.
Chametz of a Gentile
It is permitted to leave a gentile’s Chametz in one’s home over Pesach, if one does not have any liability over it even in a case of negligence, and one places it behind a Mechitza/divider of ten Tefachim [80 cm.]. This, however, only applies from before Pesach, however, on Pesach, it is Rabbinically forbidden to allow a gentile to store/leave his Chametz in one’s home, as was explained in Chapter 2 Halacha 10. [Thus, it is permitted to leave all of one’s Chametz that is included within the sale of Mechiras Chametz within one’s home over Pesach, so long as it is behind a divider that reaches the minimum height of ten Tefach. See Chapter 5 Halacha 5 for the full details of this subject!]
 Admur 443:6 “It is a mitzvah of “Hashavas Aveida” for one to sell the Chametz prior to the 6th hour, [in order to prevent his friend of a financial loss]. However, one may not do so until the 5th hour has arrived on Erev Pesach as perhaps the owner will come to get his Chametz. However, if one delays the sale until then he will not find buyers and will thus have to sell it for cheap, then one is allowed to sell it even before the 5th hour. If one is able to sell it to a gentile who he knows will return the Chametz after Pesach, then it is forbidden for him to sell it to another gentile as a complete sale.”
 Admur 443:6; See also Admur 436:3
The reason: One must destroy his friends Chametz in order so his friend not transgress Baal Yimatzeh, as all Jews are cosigners/guarantors for [the fulfillment of Mitzvos of] all other Jews, and since here one does not know for certain that the Chametz was already sold he therefore has to destroy it, as the existence of Chametz is certain. [Admur ibid and 436:3] If the Jew has liability over the Chametz, then he is obligated to destroy it in his own right, so he does not transgress Baal Yiraeh, as explained in Chapter 3 Halacha 5C!
May the owner demand payment for his destroyed Chametz: The owner cannot take the person storing the Chametz to court for not selling his Chametz prior to the time of prohibition, as the guardian is only responsible that the Chametz remain undamaged until the owner returns and not that he has to sell the Chametz for the owner. Selling it for the owner is a Mitzvah of returning a lost object and not one of obligatory responsibility for a guardian. However, if the Chametz was by him as a collateral for a loan then the borrower does not need to pay him back, as the lender has the responsibility to return the collateral back to the borrower, and since here his negligence is what led to its having to be destroyed, therefore, he lost his loan. However, if the lender kept the Chametz by him throughout Pesach, then even though after Pesach its forbidden, he can return it to the borrower, and demand payment of his loan, as no physical damage has been done to the Chametz. [However seemingly this only applies if the Mashkon did not belong to the lender, such as the collateral is worth more than the money lent.] [Admur 443:8]
 Admur 440:2 and 8
 Admur 440:9-16; See C below!
 Admur 440:5 and 8; See Halacha 10 and Chapter 5 Halacha 5!
 Admur 440:3 and 8