The Mishna on our daf discusses the case where there are two courtyards – an outer one that opens to the street and an inner one that opens to the outer one. In the event that the residents of these two hatzeirot (courtyards) put an eiruv in “one place” in order to allow them to be considered one unit – and carry in both of them – if any resident forgets to participate in the eiruv, then carrying will be forbidden in both of the hatzeirot.
Rav Yehuda in the name of Rav interprets the term “one place” in the Mishna to mean that the eiruv is placed in the outer courtyard, which is referred to in that way since it is a place that is singular in that it is available for use to members of both hatzeirot. This interpretation is supported by a baraita, which continues and teaches that if the eiruv is placed in the inner courtyard, and one of the outer residents neglected to participate in the eiruv, according to the hakhamim members of the inner courtyard can continue to carry within their hatzer, even though the residents of the outer courtyard no longer have a valid eiruv. This is because the residents of the inner courtyard can figuratively shut the door between the two courtyards and have a valid eiruv just in their hatzer. Furthermore, argues the Gemara, the residents of the inner courtyard can undo their relationship with their partners outside by saying, “We joined with you in a single eiruv to our benefit, and not to our detriment.”
A similar statement is made regarding the rules of shlihut – of sending a representative agent – in halakha. The terminology there is almost identical: “I sent you to represent me to improve my situation, not to damage it.” The basis for this is that appointing someone to represent you (and, similarly, to establish an eiruv) is predicated on the assumption that they will represent your best interests, and if they do not, the appointment is void retroactively. Obviously, such an argument cannot be applied in every case, but will only be possible to accept in a case where the shaliah does something inappropriate that was not within his purview. Similarly, in our case, it is the inappropriate behavior of the person in the outer hatzer that allows this argument to be made.