Gemara

11b (Mishnah - Chachamim): If Reuven told a Shali'ach 'give this Get of freedom to my slave', Reuven cannot retract, because Zachin l'Adam she'Lo Befanav (we may do something beneficial for someone in his absence).

(Rav Huna): We learn from Chachamim that Tefisah l'Ba'al Chov (seizing property from a borrower to acquire it for a creditor) acquires.

Question (R. Yitzchak bar Yosef): Does this apply even if this harms others? (There are other creditors who would have wanted to seize the property.)

Answer (Rav Huna): Yes!

Rejection (R. Yirmeyah): R. Yochanan taught that Tefisah l'Ba'al Chov does not acquire when this harms others.

Your proof from our Mishnah is invalid. One who says 'give' means 'acquire for.' (Reuven himself told the Shali'ach to acquire.)

(Rav Chisda): R. Eliezer and Chachamim argue about Tefisah l'Ba'al Chov when this harms others:

(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.

Objection #1 (Ameimar): Perhaps all agree that Tefisah l'Ba'al Chov does not acquire when this harms others. R. Eliezer holds that Migo (since) Levi could make his property Hefker (and become an Oni) and take Pe'ah for himself, he may acquire it for an Oni.

Objection #2 (Ameimar): Perhaps all agree that Tefisah l'Ba'al Chov acquires, even when this harms others. Chachamim expound a verse that one cannot acquire Pe'ah for an Oni.

Bava Metzi'a 10a (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 when this causes a loss to others. (Here also, others would have wanted to take the Metzi'ah.)

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

Rishonim

Rif and Rosh (Gitin 4b and 1:13): Tefisah l'Ba'al Chov does not acquire when this harms others. The Mishnah does not prove otherwise. One who says 'give' means 'acquire for.' We infer that Tefisah l'Ba'al Chov acquires when it does not harm others.

Question: Tefisah l'Ba'al Chov acquires when it does not harm others. Why can't the borrower say 'you have no claim against me'?

Answer (Rosh Bava Metzi'a 1:27 and Nimukei Yosef 5a DH u'Mihu): In a Teshuvah, the Rif says that this is only when the creditor would lose otherwise. E.g. the borrower died (a creditor cannot collect from orphans' Metaltelim). Or, the borrower was squandering his property or he was poor and it seems that nothing will remain for the creditor. In such cases, it was enacted that another can seize for him. If not, the borrower can say 'you have no claim against me.' The Rashba says that if there would be a loss, another can seize for him due to Hashavas Aveidah. The Ran says that this is even if he was not made a Shali'ach, for Zachin l'Adam she'Lo Befanav.

Rebuttal (Rosh): We infer from R. Yochanan that Tefisah l'Ba'al Chov, when it does not harm others, acquires by Torah law. It is not an enactment. It is difficult to say that R. Yochanan means that when it harms others, there is no enactment. Why would we think that Chachamim enacted to help one at another's expense?! We tried to bring a proof from the Mishnah of picking up a Metzi'ah, which is Torah law.

Answer #2 (Rosh): 'You have no claim against me' applies to claims in Beis Din. Here, someone seized. The borrower cannot demand the property back, for zl0. It is as if the creditor seized. It does not harm others when the borrower admits that he owes, or the creditor has a validated document.

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. If the borrower did not owe others, the one who seized acquired.

Question: R. Yochanan taught that Shimon can acquire a Metzi'ah for Reuven, but that Tefisah l'Ba'al Chov does not acquire when it harms others. The Gemara equated these!

Answer (Nimukei Yosef ibid.): R. Tam says that R. Yochanan does not equate them. One can acquire a Metzi'ah for another Migo (since) he could take it for himself. According to this, one creditor could seize for another.

Poskim

Shulchan Aruch (CM 105:4): If the borrower did not owe others, the one who seized acquired. This is only when the creditor would lose otherwise. E.g. the borrower was squandering his property or he was poor and lost his property.

SMA (6): The Tur mentioned seizure when the borrower was sick and about to die. The Shulchan Aruch omitted this, for nowadays, by Rabbinic enactment, we collect loans from Metaltelim or orphans.

Shulchan Aruch (ibid.): Seizure helps even before the loan was due if the borrower has no other property and the creditor will lose if not for the seizure. This is only if the creditor has a document that he could validate. If not, and the borrower denies that he owes, the property must be returned. He cannot say 'give to me time to bring a proof.'

Beis Yosef (DH v'Chosav ha'Rosh): The Rosh says that one case when the creditor would lose is if the borrower died. It should say, he is Goses (about to die). Seizure after death does not help. Or, perhaps he holds like the Mordechai (Bava Kama 201), that nowadays seizure after death helps. In a Teshuvah (106:1), the Rosh says so in the name of the Rif, and agrees.

Beis Yosef (DH Kosav Ba'al): The Ba'al ha'Itur says that if the borrower paid the Arev (guarantor), this is like Tefisah l'Ba'al Chov that does not harm others.

Hagahos Drishah (2): Even if the borrower owes others, the Arev is like a lender, and he can seize for himself.

Beis Yosef (DH Kosav ha'Rashba): The Rashba and Ra'avad hold that one who owes or has a deposit of Reuven may pay Reuven's creditor due to R. Noson's law (it is as if he owes the creditor), even if the creditor did not claim from him.

Mekor Baruch (60, cited by R. Akiva Eiger): When there is a Safek and Tefisah helps, Tefisah of a Shali'ach helps. A Shali'ach's Tefisah does not acquire only when it is surely harmful to others. If the Halachah is that he is entitled to seize, this is not harmful to others!

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.

Chelkas Mechokek (43): The Rivash explains that this is when seizure harms people who acquired (but did not receive) a gift from the Mes. If the seizure did not harm anyone, it would help, like seizure for a creditor when it does not harm others. It seems that a creditor is different, for he can force the borrower to pay in Beis Din. A widow cannot force the orphans to give more than 30 days of food at once, so also others cannot seize more than this for her and harm the orphans.

Beis Shmuel (44): Perhaps even if another seized the proper amount it is invalid. It harms the orphans, for they could have profited from the money (until she would have seized it herself).