As part of the discussion of whether a husband must actually hear the neder he is being meifer, the gemara Nedarim 72b presents a machlokes R’ Yonasan and R’ Oshiya whether an apotropus can be appointed by a husband to absolve his wife’s nedarim. R’ Yonasan takes the position that “shlucho shel adam k’moso”, but R’ Oshiya argues that the pasuk of “isha” excludes shlichus for hafaras nedarim. The gemara suggests that the possibility of the husband delegating the right of hafarah to someone else proves that the husband’s hearing the neder is not a prerequisite for hafarah.
The Rishonim ask an obvious question on this proof: if the delegate can serve as a shliach for hafarah, doesn’t the delegate hearing the neder also count as if the husband heard it by virtue of this same principle of “shlucho shel adam k’moso”?
The Ran and Rosh offer two solutions. Ran explains: a necessary condition for appointing a shliach is having the ability and right to personally perform the same act. If hearing a neder is a prerequisite to hafarah, the husband must fulfill that prerequisite before he can appoint a shliach to do hafarah on his behalf. The Rosh answers by asserting a chiddush in where shlichus applies. Shlichus, writes the Rosh, only empowers an agent to perform an action on someone’s behalf. Since hearing is passive and involves no action, is it not subject to delegation.
This Rosh is one of the proofs of the Ketzos (182:1) to his famous answer to the Tosfos RI”D’s question of why a person cannot appoint a shliach to put on tefillin for him or perfom other similar mitzvos on his behalf. If Reuvain acts as Shimon’s agent to don tefillin for him, true shlucho shel adam k'moso would mean it is as Shimon homself performed the act of putting on tefillin. However, the body those tefillin were placed on is Reuvain's, not Shimon's! Shimon's mitzvah of tefillin can only be fulfilled by putting tefillin on his own body, not someone else's. As the Rosh writes, shlichus only extends to actions - it does not mean that the agent who hears a neder functions as the ear of the husband, and it does not mean that Reuvain's body upon which tefillin are wrapped can be considered like Shimon's.