The Mishnah Berurah (444:20) writes: If one forgot to sell his chometz on erev Shabbos he needs to give it to a non-Jew as a present. He may give it to a non-Jew who he knows and trusts that will give it back to him after Pesach, however, he should do a kinyan [acquisition] that helps transfer the ownership to the non-Jew, either hagbah or meshicha [either lifting up the item, or dragging it]. If there is a large amount of chometz and the non-Jew can’t do one of these kinyanim [acquisitions], then the Jew should give over his keys to the non-Jew (a kinyan known as mesiras hamafte’ach).
If the Jew is scared to give the non-Jew a present as he fears he may not get it back, there is a machlokes in the achronim if he may sell it on Shabbos to a non-Jew. On the one hand it is not considered business as the only reason it is being done is to save oneself from the prohibition of chometz (and not for profit), on the other hand, some say selling on Shabbos even without profit is problematic.
Perhaps we can rely on the poskim who say that due to the principle of zokin le’adom sheloi befonov [I can do something beneficial for someone, even not in his presence] it is possible to sell chometz for another Jew, even without him knowing [Shu”t Chasam Sofer (Even HaEzer 1:11), Shu”t Divrei Chaim (2:46), Shu”t Be’er Yitzchok (Orach Chaim 1), and Chazon Ish (Even HaEzer 49:11)].
Based on this, the Sdei Chemed (Mareches Chometz U’Matzah 9:2) and Mo’adim U’Zemanim (3:269) say that Beis Din should write on their document of mechiras chometz, that if one of the people in the community forgets to sell his/her chometz, Beis Din are selling it on their behalf.
However, some poskim point out that it is very difficult for Beis Din to sell another person’s chometz, as one can’t sell chometz without writing the name and address of where the chometz being sold is.
Seemingly, there is another solution for one who forgot to sell his chometz, which is that he could sell his chometz to a non-Jew via a kinyan oidisa. A kinyan oidisa is a form of kinyan where the owner of the item simply says that the item now belongs to so and so, and since he is the owner, we believe him that the ownership was transferred. To do this by chometz the Jew would simply have to find a non-Jew and say that the chometz belongs to such and such a non-Jew, and the ownership would then be transferred. However, the problem with this is, that it would be difficult to get the chometz back after Pesach, as why should the non-Jew agree to give it back, and if the non-Jew doesn’t agree to return it, it would be considered stealing to eat it.
R’ Yitzchok Zilberstein brings from R’ Elyashiv a solution to this issue. He says: After Pesach the Jew should go to the non-Jew and tell him, “You should know that I have chometz by me which belongs to you, however, I refuse to tell your where, unless you are prepared to sell it to me for a small amount of money”, and certainly the non-Jew will agree.
