Gemara

(Rav): If Reuven was guarding a deposit, and he gave it to another Shomer, Reuven is exempt;

(R. Yochanan): He is liable.

(Abaye): Rav's law is not only when a Shomer Chinam gave it to a Shomer Sachar, who guards it better (he is liable for theft and loss). Rather, even if a Shomer Sachar gave it to a Shomer Chinam, who does not guard it as well, he is exempt. This is because he gives it to one with Da'as.

(Abaye): R. Yochanan obligates even a Shomer Chinam who gave it to a Shomer Sachar, who guards it better, for the owner does not want his object in others' hands.

Rav Chisda: A case occurred in which gardeners used to deposit their hoes with Leah. One day, they left them with Reuven (one of the gardeners). He heard a wedding, and left them with her. They were stolen. Rav exempted Reuven. Since they always deposited with her, they showed that they consent for her to guard them.

36b (Rava): The Halachah is, not only a Shomer Sachar who gave to a Shomer Chinam is liable, rather, even a Shomer Chinam who gave to a Shomer Sachar is liable, for the owner can say that he trusts the oath of the first Shomer, but not of the second.

Rishonim

Rif and Rosh (3:6): The Halachah is, even a Shomer Chinam who gave to a Shomer Sachar is liable, for the owner can say that he trusts the oath of the first Shomer, but not of the second.

Rosh: A Shomer Sachar who gave to a Shomer Chinam reduced the Shemirah, for the latter does not strive so hard to guard it. However, this is not negligence, for he gave it to one with Da'as. If the owner normally deposits this with the second Shomer, the first Shomer is exempt if there was no decrease in Shemirah. If he decreased the Shemirah, e.g. a Shomer Sachar gave it to a Shomer Chinam, or a borrower gave it to a Shomer Sachar, the first Shomer was negligent, and he pays, even if he borrowed or rented b'Ba'alim (the owner was working for him; normally this totally exempts). If witnesses know that the second Shomer guarded normally, the first Shomer is exempt. Even if there were no witnesses, if the first Shomer saw and can swear, he swears and is exempt. If the owner normally deposits this with the second Shomer, he cannot say that he does not trust him. If we do not know that he trusts him, even if it is known that he is more straight and honest than the first Shomer, te owner can say 'I trust your oath, but not that of the second Shomer. You cannot force me to trust one with whom I have no dealings.'

Rambam (Hilchos Nizkei Mamon 4:11): Even a Shomer Chinam who gave to a Shomer Sachar is liable, for the owner can say 'I trust your oath, but not the second Shomer's oath. Therefore, if the owner always deposited with the second Shomer, the first Shomer is exempt. He can tell the owner 'what you deposited with me, you deposited with him Emesh (last night).' This is only if he did not reduce the Shemirah. If a Shomer Sachar gave it to a Shomer Chinam, or a borrower gave it to a Shomer Sachar, since he decreased the Shemirah, he was negligent and he pays. This is even if he borrowed or rented b'Ba'alim (from his employee). This is because he removed what was guarded with himself and give it to another Shomer.

Magid Mishneh: The Rif exempts the first Shomer if he saw and can swear that Ones occurred. This is clear. When the owner regularly deposited with the second Shomer, the first Shomer is exempt even if the second Shomer went away or has no money to pay. He is exempt because the owner showed that he trusts the second Shomer. Giving to an inferior Shomer is the ultimate negligence, for the owner showed that he wanted a better Shemirah! Normally, Shemirah b'Ba'alim is exempt even for Ones. This is only for the one who borrowed b'Ba'alim, but not when he gave to another Shomer. This requires investigation.

Poskim

Shulchan Aruch (CM 291:26): If a Shomer gave to a Shomer, the first Shomer is liable, even a Shomer Chinam who gave to a Shomer Sachar. The owner can say 'I trust your oath, but not the oath of the second Shomer', even if it is known that the second is better and more Kosher than the first.

Beis Yosef (DH u'Mah she'Omar): R. Yerucham says that some argue about this. Maharik (6) says that a Shomer may not give to a Shomer even if the first is the lowliest of people and the latter is the greatest.

Gra (45): Tosfos (Bava Kama 11b DH At) exempts the first Shomer when the second is more trustworthy.

Shulchan Aruch (ibid): Therefore, if the owner always deposits this with the second Shomer, the first Shomer is exempt, if there was no decrease in Shemirah. If he decreased the Shemirah, e.g. a Shomer Sachar gave it to a Shomer Chinam, or a borrower gave it to a Shomer Sachar, the first Shomer was negligent, and he pays, even if he borrowed or rented b'Ba'alim.

Beis Yosef (DH v'Zeh): R. Yerucham (30:2 83:3) says that most Poskim challenged the Rambam about this and exempt even if he decreased the Shemirah.

Gra (46,47): Rava said 'not only a Shomer Sachar who gave to a Shomer Chinam is liable...' This connotes that this itself suffices to obligate him. Abaye exempts a Shomer Sachar who gave it to a Shomer Chinam for he gave it to one with Da'as, but this is only according to Rav. Tosfos (93b DH Iy) holds that giving to a Shomer with less liability is not negligence, for Shomrim often guard even from what they are not liable for.

Beis Yosef (DH Kosav ha'Ritva): The Ritva says that if the owner normally deposited with Ploni, and Ploni became poor or suspected, we do not exempt a Shomer who gave to Ploni.

Beis Yosef (DH v'Chosav Maharil, and Bedek ha'Bayis): Mahari Veil (31) says that if the owner deposited with David one or two times, perhaps he trusted him only in pressed circumstances. He can claim 'I did not want (him to have the deposit)'; also the Rambam mentions a Shomer that the owner regularly used. I disagree. The Rambam concludes 'he can tell the owner 'what you deposited with me, you deposited with him Emesh.'' This is even if he trusted him only occasionally. Further, according to Mahari Veil he is exempt only if he deposited with him every day, but not if he missed a day. This is absurd. The Rambam merely cited the Gemara, which said 'always'. Perhaps 'always' excludes if he deposited only in dire times. Sometimes, one who panics deposits with one whom he would not normally trust. 'Always' connotes that he deposited in calm times. Or, 'always' excludes if he used to deposit with Ploni, and Ploni became poor or suspected. 'Emesh' also connotes that he is not exempt in such a case.

Drishah (25 DH u'Mah): We learn from 'Emesh', because it is unusual for one to change to quickly (to become poor or suspected).

Beis Yosef (DH u'Mah she'Chosav): Tosfos says that when the first Shomer can swear, or there are witnesses, he is exempt, even if he gave it to a Cheresh, lunatic or minor and it died in their Reshus, for the angel of death would have killed it also in the first Shomer's house.

Shach (46): The Shulchan Aruch connotes that when there are witnesses, the first Shomer is exempt even if he gave it to an inferior Shomer. Also the Bach, Rambam and Semag say so. The Yam Shel Shlomo disagrees. I say that even an oath should help when the owner regularly used the second Shomer, even if he reduced the Shemirah. I say that the Rambam admits. He said 'as long as he did not reduce the Shemirah' to teach that if he swears that it was stolen and the first Shomer was a Shomer Sachar, he is liable. However, the Rambam says that he was negligent. The Yam Shel Shlomo explained that the first Shomer is exempt because the Ones was not due to the negligence. This exempts even if the owner did not regularly use the second Shomer. However, the primary Perush is like the Rosh, who says that this is not negligence, for the second Shomer has Da'as.