Eruvin 67A

שכח אחד מן החיצונה ולא עירב ⁠— ודאי פנימית מותרת, דאחדא דשא, ומשתמשא, וחיצונה אסורה.

But if a resident of the outer courtyard forgot and did not establish an eiruv, it is certainly permitted to carry in the inner courtyard, as its residents can close the door between the two courtyards, thereby preventing the residents of the outer courtyard from entering, and they can then use their courtyard on their own. However, it is still prohibited to carry in the outer courtyard.

אמר ליה רב הונא בריה דרב יהושע לרבא: וכי שכח אחד מן הפנימית ולא עירב אמאי שתיהן אסורות, לבטיל בר פנימית לבני פנימית, ותיתי חיצונה ותשתרי בהדייהו!

Rav Huna, son of Rav Yehoshua, said to Rava: And if a resident of the inner courtyard forgot and did not establish an eiruv, why is it prohibited to carry in both courtyards? Let the resident of the inner courtyard who forgot to establish an eiruv renounce his rights in favor of the other residents of the inner courtyard, and then let the residents of the outer courtyard, who had established an eiruv with the inner one, come and be permitted to carry together with them.

כמאן ⁠— כרבי אליעזר, דאמר: אינו צריך לבטל רשות לכל אחד ואחד? כי קאמינא לרבנן, דאמרי: צריך לבטל לכל אחד ואחד!

Rava replied: In accordance with whose opinion do you make this suggestion? It is in accordance with the opinion of R' Eliezer, who said: It is not necessary to renounce one’s rights in favor of each and every resident. Rather, it is enough for a person to renounce his rights in favor of a single person, as once he no longer has any rights in the courtyard, he can no longer render it prohibited to carry there. According to this approach, a resident of the inner courtyard may indeed renounce his rights in favor of the other residents of his courtyard. The outer courtyard would then be rendered permitted together with the inner courtyard. However, when I spoke, it was in accordance with the opinion of the Rabbis, who say: It is necessary to renounce one’s rights in favor of each and every resident. Therefore, in order to render the outer courtyard permitted, it would be necessary for the person who forgot to establish the eiruv to renounce his rights in favor of the residents of the outer courtyard as well. However, he may not do so, as one may not renounce rights from one courtyard to another. Therefore, the outer courtyard may not be rendered permitted in this manner.