Wed Nedarim 45

Tana Kama says that the partner in the courtyard that swore off pleasure from the other cannot enter the courtyard. R' Eliezer b. Yaakov says that he may, since he's going into his portion.

Ran brings the Gemara in Bava Kama that says the argument is do we say Breira. When they bought the courtyard jointly, can we say that each one owns what he eventually uses. Therefore each one walks into his possession only, since it's revealed that he was the one who bought it for that moment. Or we don't say that. When they bought it jointly, at that moment we don't know when they're going to use it, so they both own the whole property always jointly, so he's always going into what his partner also owns.

However, the Ran asks: In Beitza we Paskin that for D'oiraisa cases we don't rely on Breira, so here we should Paskin like the Rabanan that says we can't say the future reveals what he bought. However, the Gemara Paskins like R' Eliezer b. Yaakov.

The Ran answers: we only don't say Breira if we need the future to decide the whole transaction. Like if someone puts an Eiruv T'chum that it should take effect if a rabbi comes on Shabbos there and he wants to walk there. If he doesn't come, he wants the Eruv not to take effect, so he should have the same T'chum as his fellow town dwellers. Therefore we rely on Breira to reveal to us during Shabbos was there an Eiruv at here at all before Shabbos.

However, our case the transaction definitely took place, they bought the courtyard jointly. We only need to say Breira to learn the details in the purchase, which specific times did each partner buy for. Therefore we may say Breira in this case.