Gemara

98a (Mishnah): If a widow sold land worth 101 for 100, the sale is void.

99b - Question (Rav Chisda and Rabah bar Rav Huna): Is the sale valid even if the Shali'ach erred?

Answer (Rav Nachman): No. The owner says 'I authorized you to help me, not to harm me!'

(Mishnah): If Beis Din assessed property (to sell to feed the orphans or widow) for a sixth more or less than the actual value, the sale is void;

R. Shimon ben Gamliel says, the sale stands. If not, Beis Din would be weak!

Question: Whom does a Shali'ach resemble?

Answer #1 (Rava citing Rav Nachman): He is like Beis Din (if he errs less than a sixth, the sale stands).

Answer #2 (Rav Shmuel bar Bisna citing Rav Nachman): He is like a widow (any error invalidates the sale).

Rava said that he is like Beis Din because he does not sell for himself, similar to Beis Din. A widow sells for herself!

Rav Shmuel bar Bisna said that he is like a widow, since also he is an individual. Beis Din is a Rabim.

The Halachah is, a Shali'ach is like a widow.

Question (Mishnah): If a Shali'ach was told to take Terumah, he should take like the owner wants. If he does not know, he should take like an average person, one part in 50. If he took one in 40 or one in 60, the separation is valid.

Answer: Some people take Terumah generously (one in 40) and some take stingily (one in 60). The Shali'ach can say that he stimated that this is what the owner wanted to give. Regarding a sale it is a pure error. The owner can say 'you should not have erred.

Kidushin 42b (Rav Nachman): If brothers divided an inheritance and erred, i.e. some received too much and others too little, the law is like that of buyers. If the mistake was less than a sixth of the value, the sale (division) stands.

(Rava): This is when they acted themselves, but not if a Shali'ach acted for them. The Shali'ach was appointed only to help them!

Bava Basra 169b: Leah gave Levi money to buy land for her; he bought for her without Acharayos (responsibility to compensate her if it will be taken).

Rav Nachman: She sent you to help her, not to harm her! You must buy the land without Acharayos, and sell it to her with Acharayos.

Rishonim

Rif and Rosh (9: 17): The Halachah is, a Shali'ach is like a widow. Any error invalidates the sale.

Rosh: The Mishnah says that if a widow sold land worth 101 for 100, it is void. Some say that this is only if she received a Dinar too little. This is unreasonable. Since an error even less than the Shi'ur of Ona'ah (overcharging) voids the sale, it need not be a Dinar. R. Tam says that we discuss a Shali'ach appointed by Beis Din. If the owner made the Shali'ach, Rava would not say that Rav Nachman says that he is like Beis Din. Rav Nachman holds that if a (regular) Shali'ach erred, the sale is Batel, for he was authorized only to help the owner! Rava says the same in Kidushin. He holds that a Shali'ach appointed by Beis Din is stronger.

Ran (Kidushin 17a DH Aval): The conclusion is (like Rav Shmuel bar Bisna, unlike Rava,) that even a Shali'ach Beis Din is like a regular Shali'ach.

Rosh (ibid.): The Ri says that since the Gemara did not specify, we discuss a regular Shali'ach appointed by the owner. Rav Nachman said that if a Shali'ach erred, i.e. by a sixth, like in the Mishnah, the sale is Batel. In Kidushin, Rava teaches that if the brothers themselves divided, a mistake less than a sixth stands, without compensation. If a Shali'ach acted for them, it is valid, but the error is returned. Here, Rava said that a Shali'ach is like Beis Din, and the sale stands. The error is returned, because he was authorized only to help. The same applies to Beis Din; they are no stronger than a regular Shali'ach. The Rambam holds like R. Tam; this is primary. We asked regarding the Mishnah of an assessment of Beis Din. This connotes that we discuss a Shali'ach of Beis Din.

Rambam (Hilchos Mechirah 13:9): If a Shali'ach erred any amount, this invalidates the sale, whether it is land or Metaltelim. It seems that the same applies to an Apotropos. He is unlike Beis Din, for he is an individual.

Rambam (Hilchos Shutfim 1:2): If a Shali'ach transgressed the Meshale'ach's words, his action is void. The same applies if he erred by any amount, whether in land or in Metaltelim. The Meshale'ach says 'I appointed you only to help me!'

Machaneh Efrayim (Shutfim 2): The Rambam says that if a Shali'ach did not inform the seller that he is a Shali'ach, (even if he erred) the sale stands. The Rosh says that the Shali'ach bought it for himself. It seems that this is only if he was Mezid, but if he was Shogeg and made a common mistake, he does not lose. If there are witnesses that he was a Shali'ach, the sale is Batel. If not, the sale stands. The Mordechai learns like this. Some say that the Rambam holds that whenever he did not say that he is a Shali'ach, he suffers any loss due to his mistakes.

Rosh (16): Rashi says that just like when Beis Din received too little money for orphans' property the sale is void, the same applies when they received too much. The same applies to a Shali'ach. He was sent to help the Meshale'ach. Any mistake that harms the Meshale'ach is Mevatel his mission, so the same applies to a mistake that harms the other party. Rav Hai Gaon likewise says that if the Shali'ach received a sixth extra, it is Batel. Why did he mention a sixth? If it is like selling for too little, even a Dinar invalidates the sale! R. Yonah says that if the Shali'ach received too much, he is like one who sells his own property. However, buyers rely on Beis Din just like orphans do. Beis Din are Sheluchim of both parties, so any mistake of a sixth invalidates the sale. If they erred less than a sixth, the sale stands.

Ran (ibid.): In Bava Basra, Leah's Shali'ach bought land without Acharayos, and Rav Nachman forced him to buy it without Acharayos, and sell it to her with Acharayos. Why didn't he say that the sale is Batel?

Answer (Ran): There, he did not tell the seller until later that he is a Shali'ach. Therefore, the seller need not deal with anyone except for the Shali'ach; the Shali'ach must pay for it. She can say that she prefers Acharayos of the seller, and demand her money back. In any case, regarding her the sale is Batel. Rav Nachman's advised him what to do if she will want the land (with his Acharayos).

Poskim

Shulchan Aruch (EH 104:6): If a Shali'ach sold and erred and was underpaid by any amount, the sale is Batel, even if he was a Shali'ach of Beis Din. If he overcharged the buyer up to a sixth, the sale stands, and the Meshale'ach keeps the extra.

Chelkas Mechokek (12): Regarding land, even if it was sold for twice the value, the Meshale'ach keeps the extra.

Shulchan Aruch (CM 109:6): If Beis Din sold through a Shali'ach and he received any amount too little, the sale is Batel. If the Shali'ach overcharged the buyer, he is like any one (if he sold Metaltelim and the mistake was more than a sixth, the sale is invalid. If the mistake was exactly a sixth the sale is valid, but the overcharge is returned.)

Bedek ha'Bayis: It seems that the Rambam holds like R. Yonah, that if the Shali'ach received too much, he is like one who sells his own property.

SMA (24): Since the buyer bought for himself, it is no different if he bought from a Shali'ach of Beis Din or from anyone else.

Shulchan Aruch (182:3): If a Shali'ach paid any amount too much, the sale is Batel, whether he bought land or Metaltelim.

Nesivos ha'Mishpat (5): If he did not say that he is a Shali'ach, it is as if he bought it himself. The normal laws of Ona'ah apply.

Shulchan Aruch (227:30): If a Shali'ach overcharged the buyer, some say that the sale is Batel due to any amount. Some say that he is like any one, and up to a sixth is pardoned, and the Meshale'ach keeps the extra.

SMA (54,55): The first opinion holds that we equate the buyer to the seller. The latter opinion holds that since the buyer himself bought, it does not matter whether he bought from the seller or a Shali'ach.