The Gemara in Kiddushin 41b derives from a pasuk in this week’s parshah the concept of shelichus (acting on one’s behalf). The pasuk says, “kein tarimu gam atem terumas Hashem – so you too shall remove the terumah of Hashem.” The Gemara explains that the word gam (too) is superfluous; thus we draw from this that another person may remove terumah for you on your behalf.
The Gemara in Baba Metzia 22a says that, based on this pasuk, we learn another halacha regarding shelichus. The Gemara says that just as an action that you perform is with your knowledge, so too is it with your knowledge when your agent acts on your behalf. One cannot be your shaliach unless you know about it.
The Kitzos Hachoshen (105:1) asks the following fundamental question (preceded by pertinent information): There is another manner, referred to as zicheya, whereby one can act on another person’s behalf. Regarding this other form, the Gemara says that one may act on another’s behalf even without the knowledge of the other person. This is called zachin l’adam shelo befanav – one may acquire for another if it is beneficial for him, even if he does not know it. Several Rishonim opine that the mechanics behind this form of acting on another person’s behalf works with shelichus. Rashi in Gittin 9b says that whenever one is acting on another person’s behalf, it is considered as if he was appointed to be a shaliach (to do that action) for the sender. This is called zicheya mi’din shalichus. The Kitzos asks: How can zicheya work without the individual’s knowledge if it is working via the mechanics of shalichus, and shelichus requires the knowledge of the one for whom you are acting?
The Kitzos suggests that this is the reason that the other Rishonim disagree and say that zicheya is not mi’din shelichus. They say that it works instead through the halacha of yad, the fact that one’s hand acquires for him. The Kitzos goes so far as to say that even the Rishonim who say that that zicheya is mi’din shalichus do not mean that it is considered as if one made him an agent; rather, it is a gezeiras hakasuv (it works just as a shaliach works).
One could argue with the entire premise of the Kitzos. We must look at the context in which the Gemara in Baba Metzia was referring to, namely that one must be aware that there is an agent working on his behalf. The Gemara there was discussing the sugya of yi’ush shelo midas – relinquishing ownership of a lost object without the knowledge that it is lost. Rashi, at the beginning of the sugya, writes that the whole sugya is only addressing a scenario in which one would probably give up hope when he learns that the item is missing. In a scenario whereby we know for certain that one would give up hope, there would not be any machlokes and all would agree that it now works – prior to the individual actually knowing.
The Gemara brings a proof from the following Mishnah: In a case of one who takes off terumah for his friend without his friend’s knowledge, there are scenarios in which it works and those in which it is not considered as if terumah was removed. If the owner, upon hearing that his fellow removed terumah for him, responds in a manner that reveals that he is pleased, the terumah is then valid. But if his response tells us that he is upset about this, the terumah is not valid. The Gemara says that this proves that even though he did not know at the time, it is valid retroactively since he later knew about it. The same rule should apply regarding the relinquishment of ownership of a lost item.
The Gemara answers that there is no proof from this Mishnah, since the entire Mishnah is referring to a case where the owner had previously made him his shaliach.
The Gemara compares the knowledge required for yi’ush to that of a shaliach. The parallel dictates that just as if one does not know that his item is lost, even though he will probably relinquish his ownership when he learns that it is lost, we do not consider it as if he gave up hope. Similarly, even if one will probably be happy to learn that his fellow separated terumah for him, it is not considered as if he made him his shaliach since he does not presently know.