Chazakah Shaliach Oseh Shelichuso [We assume an agent fulfills his agency] — Eruvin 32a

Chazakah Shaliach Oseh Shelichuso [We assume an agent fulfills his agency] — Eruvin 32a

חזקה שליח עושה שליחותו

1. In accordance with the Gemara here, the Mishneh LaMelech in the fourth chapter of Hil. Bechoros rules that the assumption that an agent fulfills his agency is not effective immediately, but only at the end of the day. Hence, even if a person appointed an agent in the morning, he cannot assume that the agency is fulfilled until evening.
2. The Mishneh LaMelech also rules that the assumption that an agent fulfills his agency only applies in regard to a matter that the agent can complete on his own, such as to slaughter an animal or separate terumah. However, in regard to a matter which the agent cannot complete on his own, such as the sale of chametz, we are afraid that the other party might not participate in the process, and therefore in such cases we cannot assume that the agent fulfills his agency.

[Mishneh LaMelech bases this ruling on Tosafos, Gittin 64a (d.h.Assur). The Gemara there rules that if a person appointed an agent to marry a woman on his behalf and the agent does not return to identify the woman, he is forbidden to marry all the women in the world, lest they be forbidden relatives of his wife. Tosafos write that this ruling is a penalty, not the strict letter of the law — because the fulfillment of this agency requires another person's participation. Moreover, although the Ramban (ad loc.) disagrees with the Tosafos, Mishneh LaMelech explains why this argument is unique to the case of marriage and not applicable to other cases (see also Melo HaRo'im, Shaliach Oseh Shelichuso §5).]