Gemara

109b: If a Kohen is old or sick, he gives his Korbanos to any Kohen he wants. Kohanim of the division get the meat and skin.

Question: If he cannot serve, why can he choose a Kohen to be his Shali'ach? (He should need to give them to Kohanim of the division!)

Answer (Rav Papa): He can serve (and eat) with difficulty. Since such service is valid, he can make a Shali'ach. Eating with difficulty is like gorging oneself. Such eating is invalid, so Kohanim of the division get the meat and skin.

Nazir 12a (R. Yochanan): If Reuven made David a Shali'ach to be Mekadesh an unspecified woman and David died, Reuven may not marry any woman. (Perhaps David was Mekadesh her relative to Reuven!)

(Rava): R. Yochanan agrees that Reuven may marry Leah if she has no (single) daughter, granddaughter, mother, grandmother or sister. Leah is permitted even if she has a sister who was divorced (or widowed) after David was appointed. There is no concern lest David was Mekadesh her sister to Reuven, for one authorizes a Shali'ach only for something that he could do now.

12b Question (Beraisa - R. Yoshiyah): If Reuven told Ploni 'annul any vows my wife makes until I return, Ploni's annulment is invalid - "her husband will affirm her vows, her husband will annul them";

R. Yonason says, a man's Shali'ach is like himself.

If not for the verse, R. Yoshiyah would agree that Ploni can annul, even though the husband could not do this now!

Answer: He holds that the husband himself could annul now.

Kidushin 23b - Question (Rabah): According to R. Shimon ben Elazar (who says that an Eved Kena'ani cannot receive his own Get Shichrur (of freedom), can a slave make a Shali'ach to receive his Get?

Does the Gezeirah Shavah from a married woman teach that a slave can also make a Shali'ach? Or, perhaps she can make a Shali'ach for she can receive her own Get. A slave cannot receive his own Get!

Answer (Rabah): The slave can make a Shali'ach.

Question: Rav Huna brei d'Rav Yehoshua taught that Kohanim who serve in the Mikdash are Hash-m's Sheluchim. They could not be Sheluchim of Yisrael, for one cannot make a Shali'ach for something that he himself cannot do!

According to Rabah, a slave cannot receive his own Get, but he can make a Shali'ach to receive it!

Answer: A Yisrael cannot do Avodah at all, so he cannot make a Shali'ach for this. A slave can receive a Get, so he can make a Shali'ach for this;

(Beraisa): A slave can receive a Get of freedom for another slave of a different master, but not for another slave of his own master.

Rishonim

Rosh (Nedarim 10:7): If one cannot do something, he cannot make a Shali'ach for it.

Tosfos (Nazir 12a DH Mai): We conclude that even if one intends to make a Shali'ach for something he cannot do now, Shlichus takes effect only for what he could do now. How can one make a Shali'ach to knead flour and separate Chalah on the dough? Now, one cannot separate! R.Tam answered that one could bring a kneaded dough and separate now for the dough after it will be kneaded. Since it is in his power, it is considered like something he could do now.

Rambam (Hilchos Ishus 9:6): One makes a Shali'ach to be Mekadesh only a woman that he himself could have been Mekadesh at the time.

Maharit (CM 23): The Rambam's opinion is not clear. Perhaps he holds that with intent, one can make a Shali'ach for what he cannot do now. Therefore, one can make a Shali'ach to separate Chalah! Tosfos had the Rosh's text, which concludes that one cannot make a Shali'ach for what he cannot do now. The Tur and Ramban had our text (one does not make a Shali'ach).

R. Yerucham (22:3): He made a Shali'ach only for women that he himself could have been Mekadesh at the time.

Question (Mishneh l'Melech): (Hagahah: R. Yerucham connotes like the Maharit.) The Gemara applies this limitation to Shlichus to annul vows that will be made later, i.e. even though he explicitly intends for it! Can one make a Shali'ach to be Mekadesh a woman who is Safek Mekudeshes, after divorced? Now, one could be Mekadesh her mi'Safek, but not totally.

Poskim and Acharonim

Question (Maharatz Chayos Bava Kama 110a): The Mitzvah to eat Kodshim is for all the Kohanim who serve that day, not only the one who offered the Korban! Also, there is no Shlichus for Mitzvos dependent on one's body!

Question (Beis Meir EH 119:6): Some require Shelichus to write a Get. If a man does not know how to write, why can a Shali'ach write for him? According to everyone, if a sick man cannot walk to his wife who is far away, how can he make a Shali'ach to give a Get to her? The Kelal 'what one cannot do, he cannot make a Shali'ach to do it' is not only when Halachah dictates that he cannot do it. An old Kohen can make a Shali'ach only if he could serve with difficulty! Rather, one who makes a Shali'ach cannot give him more Ko'ach (Halachic strength) than he himself has. If the Meshale'ach gets more strength later, this does not help. A slave has Ko'ach, just he lacks (his own) Reshus. Therefore, a Shali'ach who has Reshus can be his Shali'ach. A Shali'ach need not receive physical Ko'ach from the Meshale'ach to transport a Get, so we are not concerned if the husband cannot walk. However, one who cannot write a Get cannot make a Shali'ach. This is difficult for those who require Shlichus to write a Get. Shlichus to divorce a wife who is now insane is valid. Even though he cannot divorce her until she becomes sane, the husband had all his Ko'ach now.

Answer (Yehudah Ya'aleh 2 EH 172): This question is not difficult if the husband can write in our script, for a Get is Kosher in any language or way of writing the letters (EH 126:1). Even one who does not know how to write at all could write if they would tear the forms of the letters or write them with spit, and he would put ink in those places, like we do for illiterate Edei Chasimah (Gitin 19a, EH 130:16). An amputee can write a Get by sitting on a printing press. This is l'Chatchilah (Taz EH 125:4 and many others). Some say that one cannot make a Shali'ach only if no one could do the act now, e.g. to be Mekadesh a woman who is currently married, or separate Chalah from flour. However, if one is merely too far away, he can make a Shali'ach. Bava Kama 110a refutes this. The Rambam (Hilchos Avodah Zarah 12:3) says that Shechitas Korban Pesach is a Mitzvas Aseh she'Lo ha'Zeman Gerama, and women are obligated. If so, why must a Mishnah teach that women can slaughter? If they could not, a Shali'ach could not slaughter for them! Kedushas Yisrael (Sof Kesuvos) exempts women from writing a Sefer Torah for this reason, i.e because they cannot write one. I say that Mitzvos are different. The Mitzvah is to do so through a Shali'ach, even though she herself cannot. Beis Efrayim (CM 17) says that if one cannot circumcize his son, Shlichus does not apply.

Question (Orach Mishur Nazir Reish 12b): We say that a slave can make a Shali'ach because he is in the Parshah of Gitin, even if he cannot receive this Get. If so, Shlichus for Kidushin should apply to be Mekadesh a woman who was married at the time, for she is in the Parshah!

Noda b'Yehudah (2 EH 69): We require that the Shali'ach could do the act. The Meshale'ach need not be able to do it, as long as he is in the Parshah. A minor cannot make a Shali'ach to receive her Get only because we do not testify about a minor. Kohanim offer Korbanos even for a weak Yisrael! The Gemara taught that a Kohen can decide which Kohen will offer his Korban only if he himself could offer it.

Matzpas Eisan (Kidushin 23b DH Rashi): One cannot make a Shali'ach for something he cannot do, i.e. when another matter is Me'akev, but he himself is qualified. E.g. a man can be Mekadesh, but he cannot be Mekadesh a married woman. A slave cannot receive his own Get (according to R. Shimon ben Elazar) due to himself (it never left his master's hand). Therefore, he can make a Shali'ach, for he himself could receive his Get if not for his problem. Now, the slave could receive a Get for another master's slave. He is unlike a sick Kohen, who cannot do any Avodah now.

Kovetz Ha'aros (72:2): Even though the Gemara in Bava Kama mentions Shlichus, it is not true Shlichus. It discusses also 'Shlichus' for eating, and surely this is not Shlichus! Rather, it is giving permission. One may give Reshus only if he could do the act himself.

Machaneh Efrayim (Shluchim 25): R.Tam holds that since one cannot circumcize Levi's son without Levi's consent, if Levi told David to circumcize him, David becomes his Shali'ach. This is like a sick Kohen who gives the Avodah to another Kohen; he is his Shali'ach. Even though Kohanim are Hash-m's Shluchim, since the Torah gave this Mitzvah to a Kohen, he can make a Shali'ach in place of himself. However, perhaps David does not become his Shali'ach, even if Levi himself could circumcize.