Mishnah 3 in the tenth chapter of Tractate Bava Metzia is the final case in the series of laws regarding a house and an upper story. We return here to a situation where the building belongs to two separate owners: one owns the ground floor and the other owns the upper story above it. The building collapsed, and the owner of the house on the bottom floor is not interested in rebuilding it. The owner of the upper story finds himself stuck, as he cannot build his apartment as long as there is nothing to support it. How, then, is this stalemate resolved?
The language of the Mishnah:
"Habayis veha'aliyah shel shenayim shenaplu" - a two-story apartment owned by two separate people, and the structure collapsed.
"Amar ba'al ha'aliyah leba'al habayis livnos, vehu eino rotzeh livnos" - the owner of the second floor demands that the owner of the first floor build his part, and he refuses.
"Harei ba'al ha'aliyah boneh es habayis vedar besecho ad sheyiten lo es yetziosav" - the owner of the upper story builds the ground-floor apartment himself and lives in it, until the owner of the house returns his full expenses. Afterward, he can continue and build the second floor.
The dispute between the Tanna Kamma and Rabbi Yehudah - 'Zeh neheneh vezeh lo chaser':
Rabbi Yehudah argues with this, and his dispute is rooted in a fundamental question found in other places in the Talmud: 'Zeh neheneh vezeh lo chaser' (one benefits and the other loses nothing). When a person benefits from his fellow, and his fellow does not incur any loss, is the beneficiary obligated to pay? For example, if a person enters his friend's house and sleeps in his basement without his knowledge and then goes on his way - must he pay rent, seeing as the homeowner lost absolutely nothing from this?
From here to our subject: The owner of the upper story moved in to live in the apartment of the owner of the house below. After he receives his expenses back, will he be required to pay rent retroactively? Although he was the one who bore the cost of the construction, from the moment his expenses are returned to him in full (and the Mishnah does not consider the time value of money), it emerges that he has been fully reimbursed, and in practice, he benefited from living in his fellow's apartment.
Everything depends, then, on the degree of benefit: If the owner of the upper story had nowhere else to live, he benefited from living in his fellow's apartment, whereas the owner of the house below - who initially did not want to build - lost nothing. This is exactly a case of 'zeh neheneh vezeh lo chaser': according to Rabbi Yehudah he is obligated to pay, and according to the Tanna Kamma he is exempt.
The approach of Rabbi Yehudah:
Due to this concern, Rabbi Yehudah takes a different position: If the owner of the upper story comes to rebuild the building, he must build both floors out of his own pocket. Once he has done so, he is permitted to go down and live specifically on the bottom floor, in order to apply pressure on the owner of the house and say to him: I am sitting in your house until you return my expenses to me. And since he also built the upper story, and could just as easily have lived there, he does not derive any tangible benefit from the bottom apartment - his sitting there is nothing but a means of pressure. Because of this, once he receives his money back, he is not required to pay rent at all.
And so we learned in the language of the Mishnah:
"Omer af zeh hadar besoch shel chaveiro tzarich leha'alos lo sachar" - one who previously lived upstairs and moved to live downstairs, once he receives the reimbursement for his expenses he will be required to pay rent for the benefit of free housing.
"Ba'al ha'aliyah boneh es habayis ve'oseh aliyah umekareh es ha'aliyah" - this is the proper course of action to be exempt from rent: to build the entire building from his own pocket, with its two stories.
"Veyoshev babayis ad sheyiten lo es yetziosav" - and then he may sit on the bottom floor until his expenses are returned to him, and once he receives them he is not obligated in rent, for he could just as well have lived in the upper story that he built.
Even though this reasoning is not simple and is not self-evident, this is the approach of Rabbi Yehudah - and the halachah does not follow him.
Halachah:
The halachah follows the Tanna Kamma. Generally, the principle is established that when "this one benefits and this one does not lose" - he is exempt. However, this is only the first stage, as there is an additional rule: if a minor loss is caused to the other person, meaning that he does lose slightly, the beneficiary is obligated to pay. Seemingly, this would apply in our Mishnah, since the owner of the upper floor might cause wear and tear to the apartment and decrease its value, for example by blackening the walls due to cooking and the like.
In practice, the Shulchan Aruch rules in accordance with the Tanna Kamma here as well, because the responsibility of the owner of the lower floor, as a partner in owning the building, was to rebuild his lower apartment for the owner of the upper floor. Because of this, the owner of the upper floor may enter and live there without any concern, and even if he causes some wear and tear in the apartment, resulting in a minor loss to his fellow - he is not obligated to pay him.
In summary: In this Mishnah, we learned that the owner of the upper floor may rebuild the ground-floor apartment himself and live in it until his expenses are repaid. The Tanna Kamma and Rabbi Yehudah disagree regarding this based on the principle of "this one benefits and this one does not lose": according to Rabbi Yehudah, he must build both floors, and only then is he exempt from paying rent; whereas according to the Tanna Kamma, whose opinion the halachah follows, he is exempt from rent, since the responsibility to build the lower floor rests upon its owner from the outset.