The gemara (Pesachim 5b) discusses whether one violates bal yera’eh and bal yematzei (BYBY) by simply accepting responsibility (kabalas achrayus) for another person’s chametz on Pesach (as opposed to having a complete ownership of the chametz).
What type of responsibility must one accept upon himself in order to transgress BYBY on Pesach? Beginning the discussion, the Rosh (1:4), Tosfos (Shavuos 44a) and the Rambam (Hilchos Ch”uM 4:3) all write that ‘kabalas achrayus’ is at least like a shomer sachar. This is based on the fact that the gemara writes in the name of Rava that one who accepts responsibility for the chametz will be liable if it gets lost or stolen. A shomer sachar is liable for theft or loss of the object being guarded.
Furthermore, Rabbeinu Dovid adds that the type of responsibility one must accept to be in violation of the prohibition of BYBY must be like a shomer sachar since a shomer chinam is only liable if there would be negligence (peshiya) involved, and therefore no one could argue that the chamtez that he is guarding would be considered his (“shelcha”). The Torah tells us that on Pesach the chametz must be “shelcha”. The chametz is not considered yours if your only responsibility is to avoid a loss due to negligence. In a different vein, the Meiri writes that peshiya is insufficient to make the chametz considered as if it were yours, since peshiya is considered like an act of damage (hezek).
In fact, the Rambam writes this explicitly (Hilchos Sechirus 2:3)-, “poshea mazik hu.” Based on this understanding, the Rambam writes that although ordinarily a shomer is patur for damage regarding shtarot/avadim, if they have committed a pshia they are chayav.
This premise is questioned by the Ravad, who points out the fact that although a shomer chinam is chayiv for hezek b’baalim (if he borrowed it in the presence of the owner) he is patur for pshia b’baalim. This clearly demonstrates that pshia is not a form of hezek as the Rambam portrayed it to be. Therefore, how can poshea be considered an act of damage!? There is no exemption of damage in a case of baalav imo! (See Birchas Shmuel Bava Kama siman 32, Bava Metzia siman 47).
In contrast, the Ritva writes that simply being a shomer chinam is sufficient to transgress BYBY. If this is true, why does Rava say that one will be liable for theft or loss? The Rosh writes that this statement of Rava refers to theft or loss that results from negligence. Additionally, the Ritva writes that it is possible that this detail of the case was only part of the story and not to be understood as a necessary component to be liable.
Tosfot (Bava Meztia 82b, Shavuos 41a) offer a third opinion and writes that one needs the kabalas achrayus of a sh’oel. Rava’s statement meant that one would be liable for theft or loss despite all precautions (even b’ones). Tosfos must hold that only when there is a liability for loss or theft despite all precautions, the shomer becomes like the owner of the object.
With regard to this chametz after Pesach, the Ritva writes that it is permitted because even though the Jew will violate BYBY, it is still called ‘chametz shel nachri’, chametz belonging to the non-Jew. The entire prohibition against chametz she’avar alav ha’Pesach only applies to chametz belonging to a Jew.
Rav Yonasan Sacks Shlit”a suggests that it is not coincidental that the Ritva writes this and also maintains that all you need to transgress BYBY is a level of shomer chinam.
You don’t need the chametz to be genuinely yours in order to transgress BYBY. All you need is to be rotzeh b’kiyumo, desiring the chametz’s presence. This level of rotzeh b’kiyumo is insufficient to give the chametz a status of chametz she’avar alav ha’Pesach. On the other hand, the Rambam (Hilchos Ch”uM 4:3) disagrees and writes that if one has chametz of a non-Jew in their possession without kabalas achrayus, then the chametz is permitted to the Jew after Pesach. This implies that if one is mekabel achrayus then the chametz is forbidden due to the prohibition against chametz she’avar alav ha’Pesach. If one is mekabel achrayus then it becomes like it is yours.
This is consistent with the Rambam who writes that you need a heightened level of shemirah, and thus a shomer chinam is insufficient. Once you have a heightened level of responsibility and ownership, it makes sense that for such an object one will be in violation of chametz she’avar alav ha’Pesach.