This Mishnah deals with partners who jointly own real property and who have taken vows forbidding one another to derive benefit from each other. The question under discussion is how they must conduct themselves with regard to the property they own together.
The wording of the Mishnah and the dispute among the Tanna'im:
"Shutafin shenadru hana'ah zeh mizeh - asurin likanes lechatzer" - two partners who vowed not to derive benefit from one another are forbidden to enter the courtyard they share, since half of the courtyard belongs to the person from whom they are forbidden to derive benefit.
Rabbi Eliezer ben Yaakov disagrees with the first Tanna and says: "zeh nichnas letoch shelo v'zeh nichnas letoch shelo" - each one of them enters his own place, and he is not entering his fellow's domain.
The basis of the dispute - the principle of bereirah:
The concept of bereirah deals with a situation in which two people are partners in a single item, and it is established that this one owns half and that one owns half, yet there is no definition of which half belongs to each of them; the division is made by dividing up the time - when this one uses it, it is his, and when that one uses it, it is his. The question is whether we say bereirah, that is, whether the matter is retroactively clarified:
If we say bereirah - as Rabbi Eliezer ben Yaakov holds: at the moment one of them enters the courtyard, it becomes clarified retroactively that he entered his own courtyard and not his fellow's, and when his fellow enters, he is entering his own courtyard. This is the very nature of the partnership from the outset: each of the partners has time that is assigned to him.
If we do not say bereirah - as the first Tanna holds: with every single entry the person is also entering his fellow's portion, for every part of the courtyard is half his fellow's, and it turns out that he is deriving benefit from him.
What is forbidden according to all opinions:
The Mishnah continues: "u'shneihem asurin l'ha'amid sham reichayim v'tanur u'l'gadel tarnegolim" - here all agree that partners who took vows against one another may not set up a millstone for grinding grain in the courtyard, nor an oven, nor raise chickens there. The reason is that these are activities in the courtyard about which a partner is entitled to object and to prevent his fellow from doing them. It is not something self-evident that the very definition of the partnership obligates him to permit; rather, it is up to him to permit or not to permit, and once he permits, it turns out that he is providing his fellow with benefit.
When only one of them took the vow:
Until now the Mishnah has dealt with a case in which each of the partners is forbidden to derive benefit from the other. Now the Mishnah discusses a case in which only one of them vowed not to derive benefit: according to the first Tanna, the one who vowed may not enter the courtyard, since he is also entering his fellow's portion. Rabbi Eliezer ben Yaakov, however, maintains his position here as well, that he can say to his fellow: I am entering my own, and I am not entering yours.
Nevertheless, the one who vowed is compelled to sell his portion to his fellow. The reason is that according to the first Tanna, who holds that the one who vowed may not enter the courtyard, there is a concern that when he sees his fellow entering he will want to enter as well, and he will come to transgress his vow. Therefore he is obligated to sell his portion.
But if it was his fellow who forbade him the benefit by means of a vow, he is not compelled to sell his portion. For if that were not so, anyone who wanted to force his partner to sell him his portion would simply vow that his partner may not derive benefit from him, and thereby force the sale. The compulsion therefore applies only when the one who is forbidden to derive benefit is the one who took the vow, and since he brought this upon himself, he must sell his portion.
In summary: in this Mishnah we learned the law of partners who vowed not to derive benefit from one another: according to the first Tanna they are forbidden to enter the shared courtyard, and according to Rabbi Eliezer ben Yaakov each one enters his own - and their dispute depends on the question of bereirah. We further learned that according to all opinions both of them are forbidden to set up a millstone or an oven in the courtyard or to raise chickens there, because granting such permission is itself a benefit. Finally, we clarified the law that when only one of them took the vow, the one who vowed is compelled to sell his portion, but when it was his fellow who forbade him, there is no compulsion, so that a person should not be able to force his partner to sell him his portion by means of a vow.