Gemara

87b (Mishnah): If Reuven sent his son (with a flask) to a grocer (Shimon), who gave to him oil, and the child broke the flask, Shimon is liable;

R. Yehudah exempts him, for he did like Reuven wanted.

Chachamim agree that if the child held the flask and Shimon measured into it, he is exempt.

Question: Why do Chachamim obligate him? Reuven knowingly gave it to his son (who is prone to break it)!

Answer (Rava and R. Zeira): The case is, Shimon took it to measure for others. R. Yehudah holds that Sho'el she'Lo mi'Da'as is like a borrower. It suffices to return it to where he took it from (the child). Chachamim hold that Sho'el she'Lo mi'Da'as is like a thief, and he must return it to the owner.

Bava Metzia 40b (Seifa of Mishnah): If one deposited a barrel with Shimon, and designated where it should be kept, whether it broke while Shimon was holding it or after he put it down, if he picked it up for his needs, he is liable.

41a (Rav Sheshes): Shimon moved it (to use it like a ladder) in order to bring chicks (from a high nest, and put the barrel down elsewhere)

Rav Sheshes says that Sho'el she'Lo mi'Da'as (one who borrows without permission) is considered a thief. The Seifa is like R. Yishmael, who holds that a thief can return the theft to where he stole it from.

R. Yochanan says that (even though the Reisha is R. Yishmael,) we must say that the Seifa is like R. Akiva, who holds that a thief must inform the owner when returning theft, for 'put it down' connotes in its place.

Rishonim

Rif and Rosh (Bava Basra 44b and 5:20): Chachamim hold that Sho'el she'Lo mi'Da'as is a thief, therefore, it is in Shimon's Reshus. Even if he returned it to the child, he is not exempt until he returns it to the owner.

Rif and Rosh (Bava Metzia 23b and 3:20): The Halachah is that Sho'el she'Lo mi'Da'as is a thief.

Nimukei Yosef (23a DH Masnisin (2)): If one lifted it with intent to use it, he is Sho'el she'Lo mi'Da'as, i.e. a thief, even before using it.

Rambam (Hilchos Gezeilah 3:15): If Levi's child or slave was holding a flask, and Ploni took it and used it, he is a Sho'el she'Lo mi'Da'as, who is like a thief. It is in his Reshus to be liable for Ones until he returns it to Levi.

Lechem Mishneh: Why did the Rambam need to say that he used it? He is liable even if he took it to use (Bava Basra 88a)!

Mordechai (Bava Kama 198): If a Shomer borrowed one item deposited with him and later returned it to its initial place, he is not a borrower. He is a regular Shomer, and [Beis Yosef's text - because] a Shomer's Da'as (intent) is like the owner's Da'as. Even R. Akiva says only that a thief's Shemirah ended, for from now and onwards the owner does not trust him, but he trusts a borrower.

Shiltei ha'Giborim (Bava Metzia 23b:1): This is even according to R. Akiva. Why isn't he considered Sho'el she'Lo mi'Da'as, who is like a thief? We must say that since a Shomer's Da'as is like the owner's Da'as, it is as if he borrowed with permission.

Shitah Mekubetzes (41a DH Tirgema, citing the Ritva):.If something is normally rented, a Sho'el she'Lo mi'Da'as intends to pay the rental, but the owner could lose from his usage. If no loss can occur, no one would consider Sho'el she'Lo mi'Da'as to be a thief. This is like Rashi, the Ramban, Tosfos and the Rashba, unlike the Ba'al ha'Ma'or.

Poskim

Shulchan Aruch (CM 292:1): If a Shomer lifted the deposit for a use that causes no loss, he is not liable until he uses it. He is Sho'el she'Lo mi'Da'as, who is like a thief.

R. Akiva Eiger: The Nimukei Yosef obligates even before using it! Machaneh Efrayim says that this is only when he removes it from the owner's Reshus.

Rema: If he returned it to the place from where he took it, he resumes his law of a Shomer, since he was a mere borrower.

SMA (4): If he took it for a use that diminishes it, he is Shole'ach Yad, which is like a thief. A thief is not trusted, so it is not considered returned without the owner's knowledge. However, the Mordechai says that we discuss something that the owner is not particular if people use it without permission. In such a case he is not called a thief. Here we do not discuss this, for he is called a thief!

Shach (1): The Rema's law is difficult. The Rif, Rambam, Rosh, Tur and Mechaber (Sa'if 6) rule like R. Yochanan, who establishes the Seifa like R. Akiva. The Rema learns from the Mordechai, but this is wrong. He understood that the Mordechai discusses one who borrowed without the owner's Da'as. If so, surely he is a thief! Also, if so the Mordechai should have said he took an item, not borrowed. Rather, the Mordechai teaches that he is not like a borrower who must return to the owner himself. Since the owner gave permission, his Shemirah did not end. The SMA (4) and Bach say that the Mordechai discusses something that the owner does not mind if people use it without permission. Even according to this, the Rema is difficult, for he connotes that we do not discuss this, for he calls him a thief, like the SMA says. The SMA and Bach say that if the owner does not mind if people use it without permission, he is not called a thief. One need not say so. He is a Sho'el she'Lo mi'Da'as, and can benefit from it when he wants. This is like money (for which he is liable for Ones)! Shiltei ha'Giborim is astounding. Rav Sheshes explicitly considers a Shomer who borrows without permission to be a thief. He is liable for Ones even after returning it. The Mordechai cannot say that R. Yochanan disagrees. R. Yochanan explained the Mishnah unlike Rav Sheshes only because he holds that 'put it down' connotes in its place. Also, in Sa'if 6 the Rema agreed with the Shulchan Aruch and Poskim. To resolve this, Ir Shushan explained that Sa'if 6 discusses use with intent to diminish it. This is wrong. Also Sa'if 6 discusses use without intent to diminish it. Until we find a proper resolution, we cannot rule like the Rema here.

Taz: We could say that the Mordechai holds that people do not consider Sho'el she'Lo mi'Da'as like a thief, therefore his Shemirah does not end (the owner consents that he remain a Shomer). However, this would not explain the Rema in Sa'if 6! Rather, we must say that he discusses borrowing with the owner's permission.

Rema (ibid.): Regarding coins, he is liable for Ones in such a case.

SMA (5): This is because coins are always ready to buy merchandise.

Shulchan Aruch (6): If one deposited a barrel with Shimon, whether or not he designated a place for it, and Shimon moved it for his own use and it broke, he is liable, whether it broke before or after he returned it to its place.

Rema (308:7): If Ploni took David's donkey without permission and used it with intent to pay rental, he is Sho'el she'Lo mi'Da'as, who is like a thief. If the donkey is normally rented, he is not a thief, unless David's household protested that David needs it. If Levi did so to avoid a loss, and will compensate David for his loss, he is not a thief, for Beis Din stipulated about this.

Terumas ha'Deshen (316): Reuven used to rent out his horse. Shimon needed to borrow it to save a big loss. Reuven's wife said 'Reuven needs it today to avoid a (smaller) loss.' Shimon said that he will pay the rent and compensate Reuven if Reuven loses. The horse died b'Ones while Shimon was using it. It seems that he is liable. Since Reuven's wife said that Reuven does not want to rent it today, Shimon is a Sho'el she'Lo mi'Da'as, who is a thief. Even though Reuven did not want to rent it only lest he lose, and Shimon agreed to pay for any loss, Bava Kama 60b forbids to take barley with intent to pay back lentils. The Rosh explains that lentils are better; still, he is called a thief. However, many hold that he is not a thief, for Beis Din stipulated that Yosef may force Moshe to help save Yosef from a loss (Bava Kama 81b). If so, all the more so Yosef may take what he needs to avoid a loss! The Rif rules that there is no such enactment.

SMA (14): The Rema (264:5) wrote that some argue about this enactment. Here he just says that Levi is not a thief, for perhaps if David were here he would agree to help save David's loss.

Ketzos ha'Choshen (3): Levi is liable if it died from work. Normally, a renter is exempt for this, but here he rented she'Lo mi'Da'as. In such a case the owner may demand the rental or the depreciation (Bava Kama 97a). The Rema teaches that he is not a thief to be liable for Ones.

Shulchan Aruch (366:3): Sho'el she'Lo mi'Da'as is a thief. If Levi's child or slave was holding a flask, and Ploni took it and used it, he is a Sho'el she'Lo mi'Da'as, and it is in his Reshus to be liable for Ones until he returns it to Levi.

SMA (7): The Tur said snatched it. The Shulchan Aruch said 'took', and did not say that the child was a minor. This connotes that even if Levi's adult child willingly gave it to Ploni, he is a Sho'el she'Lo mi'Da'as and is liable for Ones, since it is not the son's, rather, Levi's.