Gemara

9b (Mishnah): If Reuven saw a Metzi'ah and told Shimon 'give it to me', if Shimon took it and said 'I acquired it myself', he acquires it. If he said this after giving it to Reuven, Reuven acquires.

(Mishnah - R. Eliezer): If Levi gathered Pe'ah (produce left for the poor) and said 'this is for Ploni the Oni (poor person)', he acquired for him;

Chachamim say, he must give it to the first Oni he finds.

(Ula): R. Eliezer and Chachamim argue in the case when Levi is rich. R. Eliezer holds that Migo if Levi would make his property Hefker, he (would be poor and) could take Pe'ah for himself, and Migo he could acquire it for himself, he may acquire it for an Oni. Chachamim hold that we may use only one Migo.

All agree that an Oni may acquire for another Oni. Migo he could acquire it for himself, he may acquire it for another.

Question (Rav Nachman): They argue in the case when Levi is poor! Everyone is like an Oni regarding a Metzi'ah (he may take it). In our Mishnah, Reuven told Shimon 'give it to me' and if Shimon says that he took it for himself, he acquires it.

If they argue about an Oni, our Mishnah is Chachamim (therefore, Reuven does not acquire). But if all agree that an Oni can acquire for another Oni, our Mishnah is not like either Tana!

Answer (Ula): Shimon said before picking it up that he picks it up for himself.

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

This is like Tefisah l'Ba'al Chov (seizing property for a creditor) when this causes a loss to others. (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'. 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 (he acquires for his employer).

(R. Yochanan): If Shimon picked up a Metzi'ah for Reuven, Reuven acquires.

Question: In the Mishnah, Reuven does not acquire (even if Shimon did not say that he picks it up for himself until later)!

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

Kesuvos 84b: Ploni owed money to Yeimar bar Chashu. Ploni died and left a boat. Yeimar made a Shali'ach to seize it for him; the Shali'ach did so. Rav Papa and Rav Huna brei d'Rav Yehoshua (who were also creditors of Ploni) saw him.

Rav Papa and Rav Huna: You seize for a creditor in a case where it harms others. R. Yochanan says that such seizure is invalid.

Rishonim

Rif (Gitin 4b): Tefisah l'Ba'al Chov does not acquire when this harms others.

Rambam (Hilchos Malveh 20:2): If someone else seized the borrower's property on behalf of one of the creditors, it does not help. Anyone who seizes for a creditor when the borrower also owes others did not acquire.

Rosh (ibid.): If Ploni seized property from a borrower on behalf of a creditor when this harms others, he did not acquire. This is even if the creditor made him a Shali'ach, like it says in Kesuvos. Normally, one's Shali'ach is like himself. However, one cannot make a Shali'ach to harm others.

Rosh (Bava Metzia 1:27): Rav Nachman and Rav Chisda hold that if Shimon picked up a Metzi'ah for Reuven, Reuven does not acquire. This is like one who seizes property for a creditor when this causes a loss to others. He did not acquire, even if he was made a Shali'ach with Harsha'ah (power of attorney). An Apotropos (overseer) acquires, for he is like the Yad of the owner. It is as if it does not harm others. We learn from Kesuvos, and also in our Mishnah; Reuven made Shimon a Shali'ach, but it does not help. The Mishnah does not prove otherwise. One who says 'give' means 'acquire for.' We infer that if one seized for a creditor when it does not harm others, he acquired.

Poskim

Shulchan Aruch (ibid.): If one seized for creditor when this harms others, he did not acquire, even if he was made a Shali'ach with Harsha'ah. An Apotropos acquires, for he is the Yad of the owner.

Beis Yosef (DH ha'Tofes): Rashi says that if a Shali'ach seized for creditor when this harms others, he acquired. Tosfos, the Ran and R. Yerucham prove from Kesuvos that he does not. The Rosh says that even a Harsha'ah does not help.

Bach (DH ha'Tofes): If Levi merely told a Shali'ach to seize for him, Levi could retract at any time, therefore the Shali'ach cannot seize and harm others (perhaps Levi retracted). If Levi made a Harsha'ah, he cannot retract. The Shali'ach acquired a gift, and he is like a creditor and can seize! Rashi discusses this case. The Rosh learns from Yeimar bar Chashu and Metzi'ah that even a Harsha'ah does not help. What is the source that there was a Harsha'ah there?! Perhaps the Rosh infers from R. Chananel, who says that an Apotropos is like the owner. If a Shali'ach with a Harsha'ah were like an owner, he would have taught this bigger Chidush.

Rebuttal (Shach 1): The Rosh learned from Metzi'ah, to which Harsha'ah does not apply. Also, Harsha'ah is only so the borrower cannot say 'you have no claim against me.' This does not apply here (for the borrower does not claim this).

Defense (Tumim 2): Tosfos Chitzoniyos (in Shitah Mekubetzes 10a DH uv'Tosfos) says that when there is a Harsha'ah, it is not called Tefisah l'Ba'al Chov. According to one version in Rashi (Bava Kama 70a DH Kivan), Tefisah l'Ba'al Chov of a Shali'ach does not work without a Harsha'ah. The Nimukei Yosef also brings this. It seems that this applies to loans, for which Harsha'ah works properly. It is only an enactment that Harsha'ah helps for other matters. Then, it does not give any extra power to a Shali'ach.

Shach: Mahara Sason (67) says that the Ritva holds like Rashi; he made a Shali'ach with witnesses. One can say 'Kim Li (it is clear to me) that the Halachah follows Rashi and the Ritva.'

Rebuttal (Shach): Surely, witnesses do not change anything. Also, it seems that Yeimar bar Chashu was appointed with witnesses!

Question (Pnei Moshe 1:91): Only a litigant can say 'Kim Li.' One who will not benefit from this cannot say so on behalf of another, i.e. a Shali'ach for the creditor! Rather, we follow the majority opinion. However, I defer to Mahara Sason, especially for an Apotropos, for his Yad is like the orphans' Yad.

Shach: In Bava Metzia, Rashi said that Shelichus helps. He did not say so in the primary Sugya in Gitin. Surely, mere speech does not make a Shali'ach, like it says in Bava Metzia. He must hire him. We find that a worker's Metzi'os go to his employer. I think that all Poskim agree that a hired Shali'ach can seize even when it harms others, like we find in Bava Metzia.

Question (Hafla'ah, Kuntres Acharon EH 93:28): An employer gets Metzi'os that his worker finds only if he said 'work for me today', for then he is like his slave, but not if he hired him for a particular job. The Yad of a Shali'ach hired only to seize is not like the creditor's Yad!

Gra (2,3): Gitin 52a says that an Apotropos can claim to benefit orphans. In Bava Metzia, it says a worker can acquire for his employer, for his hand is like his employer's hand.

Shulchan Aruch (EH 93:28): If a widow seized Metaltelim from the estate to be fed from them, we do not take them from her.

Rema: This is only if she herself seized. Seizure by a Shali'ach does not help.