Gemara

Mishnah: (The Techum of) water in pits of Olei Bavel is like that of the one who drew it.

Rav Nachman: If Shimon drew water and gave it to Reuven, it has Reuven's Techum;

Rav Sheshes: It has Shimon's Techum.

39b: They argue about whether or not one can pick up a Metzi'ah and acquire for another.

Bava Metzi'a 8a - Rami bar Chama: We learn from our Mishnah (two people holding and claiming a Talis) that if one picks up a Metzi'ah to acquire for another, it works. If not, it would be as if each holds half, and the other half is on the ground (and he does not acquire)!

Rejection (Rava): Really, if one picks up an object to acquire for another, it does not work. Here, Migo (since) he acquires (half) for himself, he also acquires for another.

Support: If Reuven told a Shali'ach 'Steal for me', Reuven is exempt, but partners that steal are liable;

The reason is, Migo that each can acquire for himself, he can acquire for his partner.

Ula citing R. Yehoshua ben Levi: Our Mishnah (9b) is when Shimon said before picking it up that he picks it up for himself. (If he said afterwards, we say that he acquired for Reuven.)

10a - Rav Nachman and Rav Chisda: If Shimon picked up a Metzi'ah for Reuven, Reuven does not acquire.

This is like one who seizes property (from a borrower) for a creditor when this causes a loss to others (other creditors would have wanted to seize the property), and this does not acquire. (Here also, others would have wanted to take the Metzi'ah.)

Question (Rava - Beraisa): A worker keeps Metzi'os that he finds. This is only if the employer hired him for a particular job, e.g. 'Weed for me today'. But if he said 'Work for me today', the employer gets it.

Answer (Rav Nachman): A worker is different. His Yad (power of acquisition) is like the Yad of his employer (his employer acquires).

Question (Rava): But Rav taught that a worker can quit his job in the middle!

Answer (Rav Nachman): Until he quits, his Yad is like his employer's Yad.

R. Chiya bar Aba citing R. Yochanan: If Shimon picked up a Metzi'ah for Reuven, Reuven acquires.

Question: In the Mishnah (9b, Reuven asked Shimon to give him a Metziah, yet) Reuven does not acquire!

Answer (R. Chiya bar Aba): In the Mishnah, Reuven asked Shimon to give it to him, he never asked him to acquire it for him.

Rishonim

Rif (Bava Metzi'a 5a): If Shimon picked up a Metzi'ah for Reuven, Reuven acquires. In the Mishnah, Reuven said 'give it to me', he did not say 'acquire it for me.'

Rambam (Hilchos Aveidah 17:3): If Shimon picked up a Metzi'ah for Reuven, Reuven acquires, even if he did not say anything. If two people picked up a Metzi'ah, both acquire it.

Rosh (Beitzah 5:12): Seder Tana'im and Amora'im rules like Rav Nachman in monetary laws, and like Rav Sheshes in Isurim (here, the question was Techumim). Also, if Shimon picked up a Metzi'ah for Reuven, Reuven acquires. According to our text and the Perush of R. Tam, Rav Nachman and Rav Sheshes argue about this. Rav Sheshes says that he acquires, even though this harms others. Migo that he could take it for himself, he can take it for others. If Shimon drew water for Reuven, Reuven acquires it 'through' Shimon, so it gets Shimon's Techum. Rav Nachman says that Shimon cannot acquire for Reuven, so Reuven gets it from Hefker, so it gets Reuven's Techum.

Rosh (Bava Metzi'a 1:16): If Shimon picked up a Metzi'ah for Reuven, Reuven acquires. All the more so if two people picked up a Metzi'ah together, both acquire it.

Question: R. Chiya bar Aba cited R. Yochanan to say that if Shimon picked up a Metzi'ah for Reuven, Reuven acquires. Elsewhere, R. Yochanan taught that one who seizes property for a creditor when this causes a loss to others does not acquire. The Gemara says that this is the reason for the opinion that Shimon cannot acquire a Metzi'ah for Reuven!

Answer (R. Tam, brought in Rosh 1:27): R. Yochanan says that Shimon can acquire a Metzi'ah for Reuven Migo that he could acquire it for himself. Likewise, if Levi owes to Yehudah and Yosef, Migo that Yehudah could seize property for himself, he may seize for Yosef.

Rosh (ibid.): The Halachah follows R. Yochanan, for Ula and Rami bar Chama hold like him.

Nimukei Yosef (DH Garsinan): Even Rava, who challenged Rami bar Chama, challenged Rav Nachman who says that Shimon cannot acquire a Metzi'ah for Reuven. This shows that Rava holds that he can acquire.

Hagahos Maimoniyos (Gezelah 17:4): The Halachah follows R. Yehoshua ben Levi and R. Yochanan, even though Rava and Rav Nachman disagree. Some say that Rava really holds like them; he merely taught that Rami bar Chama has no proof from the Mishnah. Tosfos says that Rava challenged Rami after he accepted Rav Nachman's answer.

Question: R. Yochanan himself says that one who says 'Give' means 'Acquire for' (Gitin 11b). Why doesn't he say so here?

Answer (Nimukei Yosef, ibid.): He says so only when the one who says 'give' transfers ownership of the object; for he wants to give it as soon as possible.

Poskim

Shulchan Aruch (CM 269:1): If Shimon picked up a Metzi'ah for Reuven, Reuven acquires, even if he did not say anything.

Shach (1): The SMA says that this is when Shimon said at the time that he picks it up for Reuven. The Poskim say that it is enough if he later admits that this was his intent.

Defense (of SMA - Ketzos ha'Choshen 1): The Halachah is that Devarim sheb'Lev (unspoken intents) do not count. Therefore, his intent has no effect. The Shulchan Aruch (212:8) says that if one bought a field with intent to make it Hekdesh, the intent did not work. However, it seems that he merely vowed that he will make it Hekdesh at a later time. According to R. Yerucham, even if a buyer told witnesses that he buys for the sake of Ploni it does not help, for the seller is Makneh to the one who pays him (unless told otherwise). However, if Reuven asked Shimon to pick it up and Shimon did so silently, this is Devarim sheb'Lev Kol Adam (everyone knows that he intended for Reuven), so Reuven acquires.

Rebuttal (Nesivos ha'Mishpat 1): The Rashba (Kidushin 50a) taught that Devarim sheb'Lev do not count only if they contradict what he said. If not, they count, like regarding Terumah.

Aruch ha'Shulchan (1): One who seizes property for a creditor does not acquire when this causes a loss to others. But here, since Shimon could acquire for himself, he can acquire for others. Also, taking a Metzi'ah is not called causing a loss to others; it merely prevents them from gaining. If Shimon said that he acquires for Reuven, it works even if Reuven is not here, for Zachin l'Adam she'Lo b'Fanav. Some say that it suffices if he intended for Reuven. It is not Devarim sheb'Lev because it does not contradict anything he said.

Shulchan Aruch (2): If two people picked up a Metzi'ah, both acquire it.

SMA (2,3): This is even if one picked up one end before the other picked up the other end. This is because picking up one end acquires only what is in his hand. We assume that each is happy to be a Shali'ach for the other in order that each will acquire half.

Aruch ha'Shulchan (2): It seems that if a Yisrael picked up a Metzi'ah with a Nochri, they acquired. Even though a Nochri cannot be a Shali'ach, that refers to a proper Shelichus. He can help to acquire.