Gemara

A case occurred in which Reuven said 'give to Ploni a house that can hold 100 barrels', and it was found that Reuven owns a house that holds 120.

(Mar Zutra): Reuven wanted to give a house holding 100 barrels, not a bigger house.

Objection (Rav Ashi - Mishnah): This refers to one who sells, but one who gives a gift gives all of them.

Inference: One gives generously;

Also here, Reuven gives generously. He is happy for Ploni to get a bigger house!

Gitin 13a (Rav): If a Shechiv Mera said 'give 100 Zuz of my property to Ploni', and he did not specify which money, we do not give. Perhaps he referred to a deposit of Ploni that is buried somewhere.

The Halachah is, we are not concerned lest he referred to buried money.

Rishonim

Rif: The Halachah follows Rav Ashi. One gives generously. (Ploni gets the big house.)

Nimukei Yosef (DH Gemara): The Ritva says that we cannot learn from here to other matters. Even here, if the house held more than 120, we cannot say so (that Ploni gets it). I do not understand. What was Rav Ashi's source to learn from the Mishnah that he acquires only when it holds up 120 precisely?! We do not say that he can buy a house that holds 100 and give it (in place of the house of 120), because 'give' connotes 'from my property', if possible. The Gemara in Gitin connotes like this.

Rambam (Hilchos Zechiyah 11:22): A case occurred in which Reuven said 'give to Ploni a house that can hold 100 barrels', and we found that Reuven's house holds 120. Chachamim said that Ploni acquires the house. It seems that he intended for this, for one gives generously. The same applies to all similar cases.

The Rosh (4:16) brings the Gemara.

Poskim

Shulchan Aruch (CM 253:14): If a Shechiv Mera (Reuven) said 'give to Ploni a house that can hold 100 barrels', and we did not find a house of Reuven that holds less than 120, Ploni acquires the house that holds 120.

Beis Yosef (DH Shechiv): The Rashbam explains that Mar Zutra held that Ploni receives five sixths of the house that holds 120. Reuven thought that the house does not hold more. He said 'a house that holds 100' to show that he gives a big house. It seems that this is only when it was found that Reuven owns a house that holds 120, for people can err between 100 and 120. We would not say so if it held more than 120. In the Tur, it says that it holds 150. It seems that this is a printing mistake. Even if the Tur held that the same applies even if it holds more than 120, he should have cited the Gemara like it says, and said that the same applies to 150. Also, if the same applies to 150, what is the source to say only until 150? Perhaps the same applies to 160 or 180 or more!

Bach (DH Shechiv): I say that the same applies even if Reuven had another house that holds only 100, but he said 'house Almoni', and house Almoni holds 120. We do not say that he mixed up the houses. Rather, he thought that it does not hold more than 100. People err about such things.

SMA (33): We do not that Ploni receives only an area that holds 120, and the rest is for the heirs. Rather, he said 100 to show what an important gift he gives. He was not precise, for he had no smaller house.

Taz: The Tur connotes that if Reuven had another house we would say that he intended for it, and ha'Motzi mi'Chavero Alav ha'Re'ayah. When he has no other house, why don't we say that it was a mistake? If one said 'take this wallet full with 10 gold coins', and he erred and it really has 15 gold coins, will we say that he must give all 15?! I say that here Ploni receives the entire house because it is not normal to leave over the minority. Rather, he gives everything generously, just like (when one gives, he does not keep) a path. Reuven said what a big gift he gives, according to his knowledge. Had he known how big it really is, he would have (given it all and) said that he gives an even bigger gift. A wallet is different. It has no Shi'ur. It was a mistaken gift, therefore, Ploni gets only 10 coins. Therefore, even if the house is bigger than 120, Ploni gets it. The Gemara discusses a case that occurred. If not (rather, it teaches the limit), it should have said that said that he does not acquire a bigger house, since the law was taught like an episode. I say that even if it was found that he had a house smaller than 100, he may give it, for we can say that he erred, and ha'Motzi mi'Chavero Alav ha'Re'ayah. 'One gives generously' helps only when the house is more than 100, to teach that we do not say that he gets only 100. Even if he knows that the house is less than 100, we can say that he intended for it, and he means that it is as good as a house of 100. We find this even regarding a sale (CM 218:16), but there the buyer must know how big it really is, and here the recipient need not know.

Mishneh l'Melech (Hilchos Zechiyah 10:1): If a Shechiv Mera had two bundles, each with 120 coins, and he said 'give to Ploni the bag of 100', are we concerned lest he refers to a bag that was buried? Or, does he get exactly 100? We can compare this to the gift of a house that holds 100 barrels. We conclude that one gives generously, so he gets it all. Do not say that this is when he said 'this house', for the Rashbam said that the case is that he has no other house. If he said 'this house', Ploni would get it even if Reuven had others! If we would be concerned lest the Shechiv Mera had a wallet with 100 and it was lost, we should be concerned lest Reuven has a house overseas that holds 100!

Rebuttal (Nesivos ha'Mishpat Bi'urim 11): These are different! The gift of the house connotes a full house. If he gives only the area of 100 barrels, he did not give the entire house! Therefore, we say that he erred, and thought that it holds only 100. Had he known that it holds 120, he would have given it all. There is no reason to say that he did not intend to give 100 coins out of 120. Since he said 100, presumably this is what he wants to give.

R. Akiva Eiger: Even if it was found that he had a house that holds 80 or 90, we say that he erred, and he thought that it holds 100.

R. Akiva Eiger (Teshuvos 1:16-118): A case occurred in which David commanded to give a Get to his wife, Bas Shimon ha'Levi, and it was found that her father was not a Levi. The Beis Meir says that we can give the Get. I disagree. At least there is concern lest people say that the Get was Pasul. If it was through a Shali'ach, I say that there is concern mid'Oraisa that a different man (whose father- in-law was a Levi) sent it.

Ohr Some'ach (Hilchos Gerushin 1:1 DH v'Hinei b'She'alos): A similar case occurred in which David authorized a Get for Bas Yehudah, and this was not her father's name, and they were concerned lest David was merely jesting, or that he had another wife. I say that we learn from Bava Basra. Even Mar Zutra was not concerned lest he had another house, just he holds that Ploni does not receive more than Reuven said. Also in the Mishneh l'Melech's case of the wallet, we are not concerned lest he merely intends to jest. Even though we do not learn Isur from monetary laws, in pressed circumstances we are more lenient about Gitin, and we are not meticulous about the Get of a Shechiv Mera. All agree that the concern is only mid'Rabanan, so we attribute it to an error.

Note: Seemingly, regarding Gitin there is concern lest he intends to taunt her. If a healthy man said 'write a Get to my wife' (he did not say to give it), he is only teasing her (Gitin 66a). If a man sent a Get and was Mevatel it, the Gemara was concerned lest he just seeks to pain her (Gitin 32a). If the husband was often with his father-in-law, and frequently heard people call him by his name, it seems unlikely that he would err about this, unless his mind was weak due to illness.