Gemara

Zachin l'Adam she'Lo b'Fanav, v'Ein Chavin l'Adam she'Lo b'Fanav. (When a person is not here, one may do for him Zechus (something intrinsically advantageous), but not Chov (something with a detrimental side).)

Bava Basra 138a (Beraisa): If Reuven wrote [a document giving] his slaves to Levi, and Levi said 'I do not want it' [they are his slaves];

R. Shimon ben Gamliel says, since he said 'I do not want it', (Reuven or) his heirs acquire.

(Rava): The case is, Shimon acquired on behalf of Levi. Levi was initially quiet and later protested. Chachamim say that once he was silent, he acquired.

156b (Mishnah - R. Eliezer): We may be Mezakeh for (acquire on behalf of) a minor, but not for an adult. (He could acquire for himself);

R. Yehoshua says, we may be Mezakeh for a minor, and all the more so for an adult!

Kidushin 26b (Rav Yehudah): A case occurred in which a man wanted to give 100 animals. The only solution was to give them with land. He gave them with a square Tefach of land.

Inference: They were acquired with land, even though they were not on it! (They could not fit on it.)

Rejection: He gave coins to buy 100 animals.

Support: Had he given animals, he could have given them through Chalipin!

Objection: Also coins he could give through Meshichah!

Rather, you must say that Ploni was not there to do Meshichah. Likewise, Ploni was not there to do Chalipin!

Question: He could have asked someone else to acquire for Ploni!

Answer: He feared lest that person keep the gift himself.

Gitin 40b (Beraisa): If Reuven said 'I freed Ploni, my slave', and Ploni denies this, we are concerned lest Reuven gave a Get of freedom to a third party to acquire on behalf of Ploni.

Rishonim

Rambam (Hilchos Mechirah 5:7): If one was Makneh a Kli to the seller in order that the buyer acquire, he acquires, even if he was Makneh on condition to return it.

Tosfos (26b DH Hachi): Rashi altered the text to say 'also coins he could give through Meshichah! We must say that Ploni was not there. He didn't ask someone else to acquire for Ploni, lest that person keep the gift himself.' We support that it was a gift of coins. If not, he would have given through Chalipin. Most texts say 'if he gave coins, he could have given through Meshichah! You must say that Ploni was not there to do Meshichah. Likewise, we can say that Ploni was not there to acquire through Chalipin! Why didn't he ask someone else to acquire for Ploni? He feared lest that person keep the gift himself.' We conclude that he gave animals. R. Shemayah derives that Kinyan Sudar helps only in front of the Koneh. This is why he was not Makneh through Chalipin through someone else. The Rivan was astounded, for we always do Chalipin not in front of a gift recipient. Also, the entire Gemara connotes that this works! If it works only in front of him, why is it done through the witnesses, and with their scarf? We can explain our text according to what Rashi wrote, that no one wanted to give his scarf to acquire for the recipient. If someone was found, it would have worked.

Mahari Veil (4): Reuven was mortally sick and needed to sell his house. The buyer was his son-in-law, who was elsewhere at the time. People went to Reuven and acquired through Kinyan Sudar in front of witnesses, so he would not retract, and stipulated to fulfill the sale with all rights, and immediately sent to the son-in-law that the house was sold to him. Reuven died right away. The heirs did not want to fulfill the sale, because a document was not written. The sale is Batel. In Kidushin, some Rabanan say that Chalipin not in front of the buyer does not help. Rashi and the Ri say that it helps, i.e. for a gift, but not for a sale, in which the buyer must give money. Perhaps he does not want to spend the money! Even if he consents when he finds out, it was not clear to us at the time of the Kinyan, therefore it is void. This is like Meshichah before they agreed on a price. It is invalid, for there was no Semichus Da'as (reliance that the sale would be valid) (Bava Basra 86a). Here also, the seller had no Semichus Da'as, lest the buyer will not want to buy for this price. Even if he agrees when he hears, the scarf was already returned so the Kinyan is void (Nedarim 48b).

Question: Here, we already heard that the buyer wanted to buy for this price, so there was Semichus Da'as!

Rejection: Since the buyer can retract, this is no better than Asmachta. The seller is Makneh only if the buyer will want, so he did not resolve until the buyer heard, and the scarf was already returned.

Rashbam (138a DH Kan): When he initially protests, he did not acquire. We are not Zocheh to someone b'Al Korcho. It is a Chov for him, for Sonei Matanos Yichyeh.

Rashba (Kidushin 23a DH ul'Inyan): It seems that freedom is Zechus for a slave. A monetary gift is different, for it is not absolute Zechus. There is Chov due to "Sonei Matanos Yichyeh" (one who hates gifts will live). We call it Zechus, because people desire it. Therefore, even though it is Chov, Zachin Lo she'Lo Befanav.

Poskim

Shulchan Aruch (CM 195:3): If one was Makneh a Kli to the seller in order that the buyer acquire, he acquires, even if he was Makneh not in front of the buyer, even if it was on condition to return it.

SMA (10): This is due to Zachin l'Adam she'Lo Befanav. This is why the Rema (below) says that it is only for gifts and similar matters.

Gra (15): In many places, the Gemara says 'they acquired from him', i.e. the witnesses [did Kinyan Sudar].

Gra (16): A Mishnah (Bava Metzi'a 12b) permits writing a loan document not in front of the lender, i.e. a Shtar Aknaisa. The Rif (6a) explains that this is a Shtar Kinyan. (The borrower gives to the borrower a lien on his property from when it is written, before the loan was given.) In Kesuvos (102b) and Gitin (51a) we say that [if Levi agreed to feed daughters that his wife will have from him, and he died, they are not fed from property that he sold. Since they were not yet born, acquisition would not help for them. This implies that] if they were born already [acquisition would help, even not in front of them]. The classic text in Kidushin asks 'if he gave coins, he could have acquired through Meshichah! Ploni was not there... (I.e. we defend the answer. Had he given animals, Chalipin would have worked, even though Ploni was not there - PF.)

Rema: Therefore, the custom is to make Kinyan (Chalipin) with the witnesses' scarf, even in front of the buyer, because most Kinyanim are not in front of the buyer.

Gra (17): Even if we find that the buyer wanted at the time of the sale, since he could have retracted, this is like Asmachta. We say that any [promise starting with] 'if' does not acquire (Bava Metzi'a 66b).

Rema (ibid.): This refers to a gift and similar matters, which surely the buyer desires. Regarding a sale, perhaps he does not want to buy, therefore both of them can retract.

Darchei Moshe (6): Mahari Veil says that only a gift works not in front of the buyer. Perhaps even for a gift, he does not want to acquire, for Sonei Matanos Yichyeh. Therefore, gifts and sales are the same. He acquires only if we know that the buyer wants.

The SMA (11) cites this in the name of Mahari Veil.

Ketzos ha'Choshen: Darchei Moshe said so, but Mahari Veil never said so. He said only that some say that Chalipin never works not in front of the buyer. [All agree that] other Kinyanim work not in front of the buyer for gifts, like we find in Gitin 40b.

SMA (ibid.): The Tur and Shulchan Aruch (171:10) brought two opinions about this (whether we say that a gift is Zechus). The Rosh and Tur rule that we are not concerned for this. The Rema wrote Stam [that Zachin works for a gift]. He means that we find a Kinyan not in front of the buyer only regarding a gift, according to the opinion that this works, but no one says that a sale works. The Darchei Moshe concludes that both of them acquire only if we know at the time of the Kinyan that he wants. The custom is that the buyer sends witnesses to acquire from the seller. We need not be concerned lest he retracted, since they are his Sheluchim.

Rebuttal (Ketzos ha'Choshen 2): All agree that a gift is Zechus. This is explicit in Bava Basra 138a and 156b! If a gift were Chov, surely silence would not help! The Rambam (Hilchos Shechenim 1:5) and [one opinion in] Shulchan Aruch (171:10) say about dividing a Chatzer [that Ploni can refuse to split property because half is not useful. Even if the other party offers to give Ploni a useful amount,] Ploni can say 'I do not want a gift, for Sonei Matanos Yichyeh.' When he explicitly protests, we are not Zocheh to him b'Al Korcho. Stam, it is Zechus, and Zachin Lo she'Lo Befanav. The Rashbam and Rashba say so.