Gemara

45b (Mishnah): If each of two partners (in a courtyard) vowed not to benefit from the other, neither may enter the courtyard;

R. Eliezer ben Yakov permits them, since each enters his own portion.

Both partners are forbidden to set up in the courtyard a mill or oven, or to raise chickens;

If only Reuven was Mudar Hana'ah from his partner, he may not enter the courtyard;

R. Eliezer ben Yakov permits him, for he can say that he enters his portion;

We force the Noder (the one who vowed) to sell his portion.

46a - Question: Chachamim and R. Eliezer ben Yakov (surely) argue when each partner vowed not to benefit from the other. Do they argue when each vowed that the other should not benefit from him?

Perhaps Chachamim agree that in such a case, they are permitted, for this is Ones (each was forbidden against his will);

Or, perhaps they argue even when each forbade the other!

Answer #1 (Mishnah): If only Reuven was Mudar (i.e. Shimon forbade him, he is forbidden).

Rejection (and Answer #2): It should say (Tosfos; Ran - 'Mudar' can also mean) that Reuven was Noder (forbade himself).

Support (for Answer #2 - Mishnah): We force the Noder to sell his portion.

Granted, if Reuven himself vowed, we force him to sell his portion. But if Shimon forbade Reuven, why must Reuven sell? He is blameless!

Opinion #1 (Rabah): They argue only when the field has Din Chalukah (either of them can force the other to divide it, for each will receive a proper size field). If not, all agree that they are permitted.

Question (Rav Yosef): A Beis ha'Keneses does not have Din Chalukah, yet they are forbidden!

(Mishnah): Property of the city is forbidden to them.

Opinion #2 (Rav Yosef): They argue only when the does not have Din Chalukah. If it has Din Chalukah, all agree that they are forbidden.

(Rav Huna and R. Elazar): The Halachah follows R. Eliezer ben Yakov.

Bava Kama 51b (Ravina): R. Eliezer ben Yakov holds that Yesh Breirah. When Shimon enters, he enters his half;

Chachamim hold that Ein Breirah. When Shimon enters, he benefits from Reuven's half.

Rishonim

The Rif brings the Gemara verbatim.

Question: Why did the Gemara ask about when each vowed that the other should not benefit from him? A Mishnah (47a) teaches that if Reuven told Shimon 'I am Cherem to you, and you to me', neither may benefit from the other, or from matters of the city. If Chachamim agree when Shimon was Mudar, that Mishnah is unlike either opinion in our Mishnah!

Answer #1 (brought in Ramban): The Seifa (neither may benefit from matters of the city) refers back to our Mishnah, when each vowed from the other. When one said 'I am Cherem to you, and you to me', both are forbidden things that they own jointly and have Din Chalukah, or things owned totally by one of them. Our Gemara asked, because the Mishnah can be explained either way.

Rebuttal (Ramban): Surely, the Seifa refers what was taught immediately beforehand (one said 'I am Cherem to you, and you to me.') The Gemara would have brought the Mishnah to settle our question, if to reject the proof we must chop the Mishnah like this!

Answer #2 (Ramban): The question was, perhaps 'matters of the city are forbidden to them' refers to when each vowed from the other, but when each forbade the other. The Gemara teaches that we force a Noder who is Nadur (forbidden to benefit) to sell his share. Since he forbade himself, we are concerned lest he do something that even R. Eliezer ben Yakov forbids, e.g. setting up a millstone or raising chickens. We do not find that we force a Madir (who forbade his partner) to sell, and all the more so a Mudar (who did not vow himself), who is blameless. The Yerushalmi explains oppositely.

Rambam (Hilchos Nedarim 7:4): If two were forbidden to benefit from each other and they were partners in a Chatzer:

If it has Din Chalukah, they may not enter until they divide it. Then, each enters his part;

If it does not have Din Chalukah, each may enter his house. He says ' I enter my property.'

In either case, neither may set up a millstone or raise chickens in this Chatzer.

Rambam (5): If one vowed 'you may not benefit from me', we force the Noder to sell his share. If one vowed 'I may not benefit from you', he may enter his house, for he enters his own property. He cannot use the Chatzer in the ways we explained above.

Rebuttal (Ra'avad): The Gemara says oppositely! If one vowed 'you may not benefit from me', we do not force the Mudar to sell his share, but if one vowed 'I may not benefit from you', we force him to sell his share.

Rosh (5:1): We force the Noder to sell his share. Even R. Eliezer ben Yakov agrees. Even though he permits the Mudar to enter, we are concerned lest he do a forbidden usage, e.g. setting up a millstone or raising chickens. The Noder must sell his portion, whether he forbade himself or forbade his friend. The Gemara said that the Noder must sell to dispel the proof that they argue also about a Mudar. This is a mere Dichuy. Really, we hold like Ravina, that they argue about Breirah. Therefore, they argue even when one forbade the other. We need not change the text of the Mishnah. When one forbade the other, we force the Noder to sell his half, for he acted improperly. Just like we force one who forbade himself to sell, lest he transgress, we are concerned lest he cause his partner to transgress. The Gemara tried to prove from the Mishnah that they argue even about a Mudar. We understood that we force a Mudar to sell, not due to a fine, rather, lest he transgress. The Gemara rejects this and says that the Mishnah refers to a Noder. It does not change the text of the Mishnah. The Yerushalmi supports this. It says that we are more inclined to fine one who puts a Michshol (opportunity to transgress) in front of another than to fine one who puts a Michshol in front of himself. However, the Bavli fines both. The Rambam rules like the Yerushalmi. The Ramban says that we fine only one who puts a Michshol in front of himself. I say that we fine both. A Tosefta teaches that if two partners forbade each other and one of them often does so, we force that one to sell his share. This is when he said 'my Chatzer'. If he said 'this Chatzer', the other is always forbidden (so it does not help to force him to sell).

Ran (46a DH Iy): If he vowed twice, the Yerushalmi calls this 'often'.

Rosh (ibid.): If the Chatzer has Din Chalukah, even R. Eliezer ben Yakov forbids. We do not rely on Breirah because there is a solution through division. The Rashba says that it is because one benefits the other by not forcing him to divide and thereby allowing him to use the entire Chatzer.

Ran (46a DH Hayah (1)): We force one to sell only when he is forbidden and his partner is permitted, for then he is prone to envy his partner and come to use the Chatzer. When both are forbidden we are not concerned for this. Likewise, when one forbids his own property to himself we are not concerned.

Poskim

Shulchan Aruch (YD 226:1): If two partners in a Chatzer vowed not to benefit from each other, if the Chatzer has Din Chalukah, the vow takes effect. Neither may enter until they divide it. Then, each enters his part.

Shach (2): We do not rely on Breirah to permit, because there is a solution through division.

Shulchan Aruch (ibid.): If it does not have Din Chalukah, the vow does not take effect. Each may enter the Chatzer.