Gemara

(Mishnah): If Reuven rented Shimon's cow, and lent it to Levi, and it died normally, Reuven swears that it died normally, and Levi pays Reuven.

36a (Rav): If Reuven was guarding a deposit, and he gave it to another Shomer, Reuven is exempt;

(R. Yochanan): He is liable.

(Abaye): Rav exempts because he gives it to one with intellect. R. Yochanan obligates because the owner does not want his object in others' hands.

Question (against R. Yochanan - R. Aba bar Mamal - Mishnah): If Reuven rented Shimon's cow, and lent it to Levi, and it died normally, Reuven swears that it died normally, and Levi pays Reuven.

We do not say that (Reuven is liable because) Shimon does not want his object in others' hands!

Answer (R. Ami): The case is, Shimon permitted Reuven to lend the cow 'if you want.'

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

(Rava citing Rabah): If a Shomer was negligent, and the animal went to the swamp (where it is not guarded), and died normally, he is exempt.

This is even according to the opinion that exempts when the beginning was negligence and the end was Ones. The angel of death would have killed it no matter where it was.

Question (Abaye): R. Aba bar Mamal asked from the Mishnah, and R. Ami said that it discusses a renter who had permission to lend it. He did not answer that the angel of death would have killed it no matter where it was!

Answer (Rava): You understand that a Shomer who gave to a Shomer is liable because the owner does not want his object in others' hands. Therefore, R. Aba could ask. I obligate because the owner trusts only an oath of the first Shomer, but not of the second. R. Aba has no question! (In the Mishnah, the first Shomer saw it die, and can swear that it died normally.)

Gitin 29a (Mishnah): If Reuven told Shimon to receive something from his wife when he gives the Get, Shimon may not make a Shali'ach. We assume that Reuven does not want his deposit to be in the hands of others.

(Reish Lakish): In our Mishnah, Rebbi taught that one who borrows or rents an object may not lend or rent it to someone else.

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. (It is not like Abaye, who obligates because the owner does not want his object in others' hands.) Therefore, if witnesses saw that the latter Shomer guarded normally and Ones occurred, the first Shomer is exempt. The owner cannot say 'I do not trust the oath of the second Shomer', for there is no oath! This is even if he says that he did not want his object in others' hands. Reish Lakish derived that one who borrows or rents an object may not lend or rent it to someone else. This is l'Chatchilah. B'Di'eved, he is not liable if he did, if we know that there was no negligence. This is why in the Mishnah, the renter need not pay. The Halachah does not follow R. Meir (78a), who says that one who deviates from the owner's desire is a thief.

Rosh: The Rif connotes that it is not enough for witnesses to see that it died b'Ones. They must know that there was no negligence or Shlichus Yad. He also connotes that if any oath was incumbent on the first Shomer, the owner can say 'I do not trust the second Shomer's oath.' There is a side to say that the primary oath of a Shomer Chinam, Shomer Sachar or borrower is 'it was stolen', 'it died b'Ones', and 'it died while working', respectively; the rest is only Agav (due to the primary oath). If so, if witnesses know that it died b'Ones, there is no oath. The Halachah follows Rava against Abaye, for this is not one of the six exceptions. R. Chananel rules like Abaye, for we hold like Rabah, who obligates when the beginning was negligence and the end was Ones. This is wrong. Rava agrees that he is liable, just he holds that giving a deposit (without permission) to one with Da'as is not negligence.

Rambam (Hilchos Sechirus 1:4): A borrower (or renter) may not lend (or rent) the item to another, even a Sefer Torah, that everyone who reads in it does a Mitzvah. The owner can say that he does not want his object in others' hands. If a Shomer gave to a second Shomer, if witnesses know that the second Shomer guarded normally and Ones occurred, the first Shomer is exempt, for there are witnesses of Ones. If there are no witnesses, the first Shomer must pay because he gave to another Shomer, and he claims from the second Shomer.

Rambam (Hilchos Nizkei Mamon 4:11): A Shomer who gave to a Shomer is liable, for the owner can say 'I trust your oath, but not the oath of the second Shomer.'

Magid Mishneh: The Rif wrote that the first Shomer is exempt also if he saw and can swear that Ones occurred. This is clear.

Rosh (1:3): Some ask an empty question, that the second Shomer should exempt the first through his testimony that it died b'Ones. Really, the first Shomer is liable because he cannot swear. He owes money. The second Shomer cannot exempt from a debt to pay money.

Shach (CM 291:46): If there is a second witness with the second Shomer, they exempt the first Shomer. However, others say that the second Shomer is not a witness, for he is partial. This is primary.

Mordechai (270): If the first Shomer can swear about the Ones, he is exempt. R. Chananel rules like Abaye, for Rav and R. Yochanan argue about when the beginning was negligence and the end was Ones, and Abaye holds like Rabah, who obligates for this. This is wrong, for even Rava exempts only when we cannot attribute the Ones to the negligence.

Poskim

Shulchan Aruch (CM 291:26): If a Shomer gave to a Shomer, the first Shomer is liable. The owner can say 'I trust your oath, but not the oath of the second Shomer.'

Shach (41): If the second Shomer was negligent, the owner can collect from either. The second cannot say 'you have no claim against me.' This is like a thief who stole from a thief. Even if the second borrowed b'Ba'alim (the first Shomer was working for him; normally, this totally exempts.), he is liable, for the item does not belong to the first Shomer.

Rema: However, if a Shomer gave to a Shomer in front of the owner and the owner did not protest, the first Shomer is exempt.

Shulchan Aruch (ibid): 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.

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.

SMA (49): He swears that the second Shomer was not negligent and was not Shole'ach Yad.

Rebuttal (Shach 48): The Rambam and Shulchan Aruch hold that the primary oath is that it was stolen. The other oaths are only through Gilgul. If witnesses say that it was stolen, there is no need for the other oaths. The same applies if the first Shomer can swear that it was stolen. The Rosh and Tur say so. The Bach requires witnesses to exempt from all three oaths. This is wrong.

Ketzos ha'Choshen (14): The SMA is correct. The Rosh says that when witnesses exempt from the primary oath, there is no Gilgul. However, when the first Shomer swears that it is not in the second's Reshus, but does not know if it was lost through negligence or Ones, since he cannot swear he must pay. This applies even to a Gilgul oath. The Shach himself (75:51) says so! The SMA obligates the first Shomer for Shlichus Yad of the second; it seems that the Rosh agrees. Granted, he is liable for negligence of the second. Why should he be liable for Shlichus Yad of the second? How can he swear that the second was not Shole'ach Yad? Must we say that the first Shomer was with the second the entire time?! The Ritva exempts for Shlichus Yad of the second, for Ein Shali'ach l'Devar Aveirah. This is difficult, for regarding Shlichus Yad, Yesh Shali'ach l'Devar Aveirah! It seems that the Halachah follows the Rosh, but the first Shomer need not swear about Shlichus Yad of the second, for all agree that it is only a Gilgul oath.