Eruvin 66B

אלא לאו, דאתא בשבתא, וקתני: אוסרין ואין מערבין, אין מבטלין. שמע מינה.

Rather, is it not referring to a case where the non-Jew arrived on Shabbat, and Shmuel is teaching: In a place where they render each other prohibited from carrying but they may not establish an eiruv together, in such a situation they may not renounce their rights for each other. Therefore, you can learn from this that if the non-Jew arrived on Shabbat, they cannot rent his property and then renounce their rights to one of them. This explains R' Elazar’s surprise at R' Yoḥanan’s ruling, as it appears to contradict this teaching of Shmuel, his first teacher.

אמר רב יוסף: לא שמיע לי הא שמעתא. אמר ליה אביי: את אמרת ניהלן, ואהא אמרת ניהלן, דאמר שמואל: אין ביטול רשות מחצר לחצר.

Rav Yosef said: I have not heard this halakha of Shmuel’s with regard to two courtyards situated one within the other, that the residents of the inner courtyard may renounce their rights to the outer courtyard in favor of the residents of that courtyard. Abaye said to him: You yourself told it to us. Rav Yosef forgot his studies due to illness, so his student Abaye would remind him of his own teachings. Abaye continued: And it was with regard to this that you told it to us. As Shmuel said: There is no renunciation of rights from one courtyard to another. In other words, while one may renounce his rights to his own courtyard for the other residents of that courtyard, he may not renounce his rights to another courtyard for the residents of that courtyard.

ואין ביטול רשות בחורבה.

Likewise, there is no renunciation of property rights in a ruin. If a ruin was shared by two houses, neither can renounce its rights to the ruin in favor of the other. The rabbis instituted renunciation of rights only with regard to a courtyard, as that is the typical case.