A Shomer Who Gave to Another Shomer

The gemara (Bava Metzia 36a) discusses a shomer who handed over the object to another to watch without the owner’s permission, whereupon the object was lost from the second shomer. R. Yochanan says that the first shomer is obligated to pay even if he ostensibly raised the level of the watching by paying the second shomer (making him a shomer sachar), whereas the first shomer was only a shomer chinam. Rav says that the first shomer is exempt if that which happened to the object does not obligate a shomer of his type.
The gemara presents two explanations of R. Yochanan’s ruling to obligate the first shomer. Abayei says that it is because the owner can say that it is against his will for his object to be in the hands of someone other than whom he entrusted. Even if the object was lost by oness (extenuating circumstances) it is a case of techilato b’p’shiya v’sofo b’oness (= tbpvsb- something that began as negligence, which obligates even if at the end there was an oness).
Rava says that while it was not negligence per se to give the object to the second shomer, the owner does not have to accept the second shomer’s oath regarding what transpired (assuming no one can prove what happened to the object). No one can swear: the first shomer does not know for sure, and the second one is not believed. Rava posits that a shomer’s oath is not valid because it convinces beit din but because it was accepted by the owner. He cannot refuse to accept the oath of the one whom he entrusted with the object, but he can refuse the oath of someone else. We accept R. Yochanan’s opinion according to Rava’s explanation, and, therefore, we obligate the first shomer unless he can prove what happened to the object. He is also exempt if it can be shown that the owner trusts the second shomer.
There are noteworthy elements in the Rambam’s presentation (Sechirut 1: 4-5). On one hand, he follows Rava. Yet he says that if the second shomer had a lower level obligation than the first, he is obligated because of negligence even if he was a shomer for the owner in the past, unless he brings evidence of an oness. This is difficult: if it is negligence to give it to the second shomer, he should be obligated no matter what, due to tbpvsb! One answer is that since the Rambam talked about an animal dying naturally, tbpvsb does not apply, especially since there was no real negligence, as even the first shomer is exempt when that happens (see Lechem Mishneh ad loc.). The K’tzot Hachoshen (291:12) explains that it is not a matter of negligence, but that even one who is sometimes a shomer for the owner does not have the rights to make an oath about what happened in regard to matters that he was not entrusted. In this case, the second shomer cannot make the oath regarding g’neiva va’aveida, which is beyond the realm of his shemira; thus, witnesses are necessary.