Gittin, Chapter 1, Mishnah 6. This mishnah rests on a fundamental principle: when an act or a document constitutes a benefit for a person, a third party may acquire it on his behalf and take possession of it in his name; but when it holds no benefit for him, no one else may act in his name unless he was explicitly appointed as his agent.
The wording of the mishnah and the view of Rabbi Meir:
"Ha'omer: ten get zeh le'ishti ushtar shichrur zeh le'avdi - ratzah lachzor bishneihen, yachzor, divrei Rabbi Meir" - If the get and the writ of manumission were considered a benefit for the wife and for the slave, the third party would acquire them on their behalf the moment they came into his hands. But in Rabbi Meir's view neither one is a benefit, not for the wife and not for the slave, and therefore the acquisition does not take effect until the documents actually reach their hands. As long as they have not reached their hands, the husband or the master may retract, since the matter has not yet taken effect.
The view of the Sages:
"VaChachamim omrim: begitei nashim, aval lo beshichrurei avadim" - The Sages agree that with a get the husband may retract, since divorce is not a benefit for the wife. But with the freeing of a slave, which is a benefit for him, the moment the document is handed to the agent the slave becomes a free man, and the master can no longer retract. The basis of this is:
"Zachin le'adam shelo befanav" - One may act on a person's behalf in something that is to his benefit and advantage even when he is not present and without his authorization.
"Ve'ein chavin lo ela befanav" - But something that is to his detriment and disadvantage, such as a get, may be done only in his presence, that is, with his knowledge, his consent and his will.
The reason for the distinction between the slave and the wife:
The mishnah explains the distinction: "She'im yirtzeh shelo lazun et avdo - rashai, veshelo lazun et ishto - eino rashai":
Regarding the slave: Even while he was a slave, his master was not obligated by law to feed him, and in any case he had to see to his own livelihood. It turns out that in receiving his freedom he loses nothing and only gains, and therefore this is a benefit for him.
Regarding the wife: As long as she is married, the husband is obligated to provide for her and may not withhold this. Once she is divorced she loses this right, so divorce brings her no advantage.
Rabbi Meir's argument:
"Amar lahem: vaharei hu posel et avdo min haTerumah keshem shehu posel et ishto" - Rabbi Meir seeks to prove that the slave too suffers a loss through his release: if the master is a kohen, his slave used to eat Terumah, and from the moment he is freed he may no longer eat Terumah anywhere - exactly as a woman who is divorced loses her right to eat Terumah.
The Gemara notes as a side point that this argument would seemingly apply only when the master is a kohen, and it explains that there is a loss even for the slave of a Yisrael: as long as he is a slave he is permitted to a Canaanite maidservant, and once freed she becomes forbidden to him. And although he is now permitted to marry a Jewish woman, a slave prefers a life of license that imposes no obligations upon him, and therefore being freed is not necessarily a benefit for him.
The Sages' response:
"Amru lo: mipnei shehu kinyano" - The slave eats Terumah only because he is the property of the kohen, not because of any inherent status of his own. Consequently, even while he was a slave the right to Terumah was never guaranteed to him, for at any moment his master the kohen could have sold him to a Yisrael and thereby disqualified him from Terumah.
Where the sender died before the delivery:
"Ten get zeh le'ishti ushtar shichrur zeh le'avdi, umet - lo yitnu le'achar mitah" - The get and the manumission must take effect through the power of the husband or the master, and once they are no longer alive the matter cannot take effect.
"Tenu maneh le'ish peloni, umet - yitnu le'achar mitah" - With a gift of money there is no need for the owner of the money to be alive at the time of the giving, since the money is given from the estate.
In summary: In this mishnah we learned the dispute between Rabbi Meir and the Sages regarding agency for divorce and for freeing a slave. According to Rabbi Meir, in both cases the sender may retract until the document reaches the hand of the recipient, while according to the Sages the distinction lies in the rule "one may act on a person's behalf for his benefit even when he is not present, but one may not act to his detriment except in his presence": freeing a slave is a benefit for him and takes effect immediately, whereas divorce is a detriment to the wife. We examined the reasoning based on support, Rabbi Meir's argument from Terumah and the Sages' response that the slave eats only because he is his master's property, and finally the difference between a get and a writ of manumission, which are not given after the sender's death, and a gift of money, which is given even after death.