Gemara

80b (Mishnah): If Reuven told Shimon 'guard this for me', and Shimon said 'leave it in front of me', he is a Shomer Chinam.

99a (Rav Huna): If Reuven borrowed an axe from Shimon, once he chops with it, he acquires it. Before chopping with it, he does not acquire it.

Suggestion: He acquire it to be liable for Ones.

Rejection: This should be no different than a cow. He is liable once he borrows it, even before using it!

Rather, he acquires the right to keep it for the duration of the loan. Before Reuven chops with it, Shimon can demand that he return it.

Rav Huna argues with R. Ami.

(R. Ami): If Levi lent a Hekdesh axe to Yehudah, Levi transgressed Me'ilah according to the Tovas Hana'ah (how much he would pay to be able to lend an axe). Yehudah may use it.

If Yehudah did not acquire it (even before using it), why does Levi transgress, and why may Yehudah use it? He should be obligated to return it to avoid Me'ilah!

Rav Huna argues also with R. Elazar.

(R. Elazar): Just like Meshichah was enacted for buyers (to acquire in place of money), it was also enacted for Shomrim.

A Beraisa says just like R. Elazar.

Bava Kama 79a (Mishnah): If Reuven was dragging Shimon's animal to take it out (and steal it), and it died while still in Shimon's premises, he is exempt. If he lifted it or took it outside, and then it died, he is liable.

If he gave the animal to... a Shomer, and he was dragging it out, and it died in his (Shimon's) premises, he is exempt.

Question (Ameimar): Was Meshichah enacted regarding Shomrim?

Answer (Rav Yeimar - Mishnah): If he gave the animal to a Shomer, and he was dragging it out, and it died in his premises, he is exempt.

Suggestion: Reuven's Shomer was dragging it. (Reuven is exempt because Meshichah was enacted for Shomrim, and it was not done!)

Rejection (Ameimar): No, Reuven was dragging it from the premises of Shimon's Shomer.

Question: We know this from the Reisha!

Answer: The Mishnah teaches about one who steals from the owner's premises, and one who steals from a Shomer's house.

Objection (Rav Ashi): There is no reason to differentiate between them! (The Mishnah need not teach both.) Rav Yeimar's answer is valid.

Rishonim

Rif and Rosh (8:15): Rav Huna argues with R. Elazar. A Beraisa supports R. Elazar.

Nimukei Yosef (57a DH b'Eved): Even if he said 'hit it (so it will come to me) and I will be liable', he is exempt (until it gets to his Reshus), because the lender can still retrieve it.

Rosh: Meshichah is so the lender cannot retract. Even without Meshichah, a borrower is liable for Ones and a Shomer Chinam is liable for negligence (80b). Likewise, a Shomer Sachar is liable.for theft or loss (without Meshichah). If a borrower said 'hit it with a stick and it will come to me', and it went to a Reshus ha'Rabim, where Meshichah does not acquire, the lender can retract, for it is no better than Meshichah there. Still, the borrower is liable for Ones. Also, the Gemara said that Rav Huna argues with R. Elazar. This connotes that R. Elazar discusses retracting the loan, like Rav Huna. There is great reason to distinguish between retracting and Onesim. Regarding retracting, it is in the owner's Reshus until the Shomer does something that acquires in a sale. Liability for Onesim comes once the owner ceases to guard it.

Rambam (Hilchos Sechirus 2:8): Just like Meshichah was enacted for buyers, it was also enacted for Shomrim. If Reuven told Shimon 'guard this for me', and Shimon said 'leave it in front of me', he is a Shomer Chinam.

Magid Mishneh: A Beraisa supports R. Elazar. He teaches that after Meshichah, the lender cannot retract. The same applies to a Shomer Sachar. Also, the Shomer is liable for Ones only after he did Meshichah. The Rashba asked how one is liable for saying 'leave it in front of me.' He did not do Meshichah! It seems that he left it in a Simta within his four Amos, or the borrower said 'hit it with a stick and it will come to me.' The Ra'avad says so. All the more so, if it is in the borrower's Reshus he is liable!

Rambam (Hilchos She'elah 1:5): If one borrowed a Kli or animal Stam, the lender can demand it back whenever he wants. If he borrowed it for a fixed time, once he did Meshichah, he acquired, and the owner cannot take it back from him until the time ends. Even if the borrower died, his heirs use it until the end of the time. This is reasonable. One who receives a gift, even though he gave nothing, acquires the object permanently, like one who buys. Similar, one who borrows, even though he gave nothing, acquires the object for the set time, like one who rents.

Rosh (Bava Kama 7:11): After Meshichah, the Shomer is liable.

Poskim

Shulchan Aruch (CM 291:5): Some say that once a Shomer accepts to guard or says 'leave it in front of me' and the owner ceased to guard it, the Shomer is liable for negligence even if he did not do Meshichah.

SMA (8): The Shomer become liable because the owner ceased to guard it. If the owner continued to guard it a little, the Shomer would be exempt, for this is Shemirah b'Ba'alim, like the Tur and Shulchan Aruch say at the end of this Siman.

Shach (12): They do not say so at the end of this Siman! Siman 346:4 (see below) connotes that this is not Shemirah b'Ba'alim!

Gilyon Maharsha (346:4): The SMA holds that since the borrower is already liable, the owner need not be concerned for his animal, so if he helps, it is to help the borrower. If the borrower stipulated that he must help, this is not b'Ba'alim, for he only accepted to guard together. The Shach holds that even though the Shomer is liable, the owner prefers his own animal to payment for it, so he helps for his own benefit.

Shulchan Aruch (ibid): Some say that he is not liable until he does Meshichah in a place where Meshichah acquires.

Shach (13): Rashi and the Rambam say that he is not liable until Meshichah. Bava Kama 79a connotes like this.

Gra (13): Tosfos and the Rosh in Bava Metzia say that he is liable before Meshichah. In Bava Kama they exempt before Meshichah, and Bava Kama 79a connotes like this. This requires investigation.

Shulchan Aruch (CM 307:2): Either party in a rental can retract until Meshichah or a Kinyan is done. Some say that once the owner releases his Shemirah over it according to the Shomer's desire, the Shomer is liable to guard it.

Beis Yosef (DH u'Mah she'Chosav): Tosfos and the Tur hold like the Rosh. The Rambam disagrees, and holds that the borrower is liable only after Meshichah.

Shulchan Aruch (341:1): If one borrowed a Kli or animal for a fixed time, once he did Meshichah, he acquired, and the owner cannot take it back from him until the time ends. Even if the borrower died, his heirs use it until the end of the time.

Shulchan Aruch (346:6): If the owner went with the animal he lent to help it and load it, it is Shemirah b'Ba'alim, even if the owner did so on his own (Rema - unlike those who disagree).

Darchei Moshe ha'Aruch (7): The Mordechai (373) holds that it is Shemirah b'Ba'alim only if the borrower asked him to work for him.