Gemara

94b (Mishnah): If David David borrowed Levi's cow and at the same time asked or hired Levi to work for him, or if he Sho'al (asked) or hired Levi to work and later borrowed it, and the cow died, David is exempt - "Im Ba'alav Imo Lo Yeshalem". If he borrowed Levi's cow and later asked or hired Levi to work for him and the cow died, he is liable - "Ba'alav Ein Imo Shalem Yeshalem."

Question: The middle clause says 'and later borrowed the cow.' This implies that the Reisha means precisely at the same time. How is this possible? Levi is hired as soon as David asks, but the cow is not borrowed until David does Meshichah (takes it to his premises)!

Answers: The cow was already in David's Chatzer, so no Meshichah is needed. Or, David asked Levi to begin working when he does Meshichah.

(Abaye): "Ba'alav Ein Imo Shalem Yeshalem" - he is liable because the owner was not working for him at either time, not the time of She'elah nor the time of death. If he was working for him at one time, he is exempt.

Question: "Im Ba'alav Imo Lo Yeshalem" exempts only if the owner was working for him at both times.If he was working at only one time, he is liable!

Answer: If he was working for him only at the time of She'elah, he is exempt. If he was working only at the time of death, he is liable!

(Rava): "Im Ba'alav Imo Lo Yeshalem" - he is exempt if the owner was working for him at both times, or even at one of them.

Question: "Ba'alav Ein Imo Shalem Yeshalem" - he is liable if the owner was not working for him at even one of the times!

Answer: If he was working for him only at the time of She'elah, he is exempt. If he was working only at the time of death, he is liable!

Question: We can say oppositely! If he was working only at the time of She'elah, he is liable. If he was working only at the time of death, he is exempt!

Answer #1: Presumably, the time of She'elah is more important, since the animal comes to his jurisdiction.

Question: No, the time of breakage or death is more important, for that is what he pays for!

Answer: He is liable for feeding it from when he borrows it.

Answer #2 (Rav Ashi): One is liable for She'elah "me'Im Re'ehu" (from his neighbor), but not for She'elah 'Im (with) Re'ehu'.

We need "Be'alav Ein Imo" and "Im Be'alav Imo". If not for them, we would say that it is normal for the verse to say me'Im* Re'ehu and not expound it.

97a (Rava): If one wants to borrow something and be exempt, he should ask the lender 'give me a drink of water.' This is b'Ba'alim. If the lender is sharp, he will tell the borrower 'I will give to you after you take the object.

(Rava): The following are considered b'Ba'alim when they are working: a teacher of children (who lends to the parents), one hired to plant a vineyard for half the Peros, a slaughterer, a blood-letter, and the town Sofer.

Rabanan (to Rava): You work for us.

Rava: No, you work for me. I decide which tractate we learn, and you cannot protest!

The Halachah is, before the festival, when the Rebbi must teach laws of the festival, he works for them. At other times, they work for him.

Rishonim

Rif and Rosh (8:1): If he was working for him at the time of She'elah, even if he was not working for him at the time of death, he is exempt.

Rosh (6): Rava (97a) says that She'elah b'Ba'alim is only if he was working at the time, but the beginning of the Perek says that it is from when he accepted to work! It seems that Rava does not mean when they are working, rather, from when they prepare and move themselves to go. The Ra'avad explains like this. If one asked to borrow Ploni and his cow, from when he starts walking with his cow, even before they leave his house, the She'elah began. He explains that 'at the time of working' means at a time that he could do that task, even if he is not working. I.e. since he is hired for the city whenever they need him, it is She'elah b'Ba'alim to the entire city, for if anyone needed him, he must work for him. This is like partners who hired someone for a month or year. It is always She'elah b'Ba'alim to the partners.

Rambam (Hilchos She'elah 2:1): If one was Sho'el b'Ba'alim, even if it was lost or stolen through negligence, he is exempt, as long as the owner was borrowed first. The same applies if the owner was borrowed or rented, whether for the same job or a different one (than the item borrowed). Even if one told his friend 'give to me water to drink' and borrowed his animal, and he give to him to drink and gave to him the animal, this is She'elah b'Ba'alim and he is exempt. If he did Meshichah on the animal and afterwards give to him to drink, it is not She'elah b'Ba'alim. The same applies to all similar cases.

Rambam (Hilchos Sechirus 1:3): If one deposited with his friend, for free or for pay or lent or rented to him, if the Shomer borrowed or hired the owner with the item, the Shomer is totally exempt. This is even if it was lost through negligence. This is when he borrowed or hired the owner at the time he took the item, even if the owner was not there at the time of the Ones. If he took the item first and became a Shomer, and after it came to his Reshus he hired or borrowed the owner, even if the owner was there at the time of the Ones, he must pay. Tradition teaches that if the owner was with him (borrowed or hired) when he borrowed the item, even if he was not with him at the time of theft or death, he is exempt. If he was with not with him when he borrowed the item, even if he was with him at the time of theft or death, he is liable. The same applies to other Shomrim. All are exempt b'Ba'alim, even for negligence.

Poskim

Shulchan Aruch (CM 346:1): If one borrowed, and the lender was with (working for) the borrower when he did Meshichah on the borrowed item, even if it was lost or stolen through negligence, he is exempt. This is whether the owner was working for free or was hired, whether for the same job or a different one than the item borrowed. Even if one told his friend 'give to me water to drink' and lent to him his cow while giving to him to drink, this is She'elah b'Ba'alim and he is exempt. If he did Meshichah on the animal and afterwards give to him to drink, it is not She'elah b'Ba'alim.

SMA (1): Ir Shushan says that the verse connotes that it depends on whether or not the owner was borrowed at the time he lent the animal. He overlooked the Gemara (which said that we could expound that it depends on the time of Ones, and needed to learn from reasoning).

SMA (2): Even if he did Meshichah on the animal and afterwards the owner did a real labor for the borrower, it is not She'elah b'Ba'alim.

Gra (3): The Rambam holds that She'elah b'Ba'alim is only after he started working. Tosfos and the Rosh hold that it is even before he started (Sa'if 3).

Shulchan Aruch (2): Whether the owner was borrowed together with the cow, or if he was borrowed or hired and afterwards lent his cow, the Shomer is exempt, even if the owner was not there at the time it broke or died. However, if he was not with him at the time he borrowed the item, and afterwards he borrowed or hired the owner, it is not She'elah b'Ba'alim, even if the owner was there at the time it broke or died.

Shulchan Aruch (3): If he asked to borrow the owner and the owner agreed, even if he did not yet begin working when Meshichah was done, just he was preparing himself to go, this is b'Ba'alim. If he just verbally accepted to work for him, but did not prepare himself for work, it is not She'elah b'Ba'alim.

Tur: The Ramah says similarly, but only if no time was specified; he may do the task whenever he wants. Since if he would say 'do it now', he would do it, the She'elah already began, even though he did not start working yet. If he told him to do a task immediately, as long as he did not begin, he is not considered to be working for him.

Bedek ha'Bayis: It seems that this is a printing mistake. For contrast with the Reisha, the Tur should conclude 'if a time was fixed, as long as he did not begin, he is not considered to be working for him.' However, this is difficult, for if so, once the time comes, he is considered to be working for him even if he did not start yet.