Gemara

(Mishnah #1): If Reuven appointed Shimon to give a Get within Eretz Yisrael, and Shimon got sick, Shimon can appoint a Shali'ach to give the Get;

Contradiction (Mishnah #2): If Reuven told two people 'give a Get to my wife', or he told three 'write and give a Get to my wife', they themselves must write and give. They cannot appoint someone else.

Answer #1 (Abaye): In Mishnah #2, the husband is insistent that they write it themselves, lest word spread that he himself does not know how to write a Get. In Mishnah #1, the husband does not care who gives the Get.

Answer #2 (Rava): In Mishnah #2 they cannot make another Shali'ach because Mili (a mission in which the Shali'ach did not receive anything needed for the mission, or a mission that depends on the consent of someone else) Lo Mimseran l'Shali'ach (cannot be transferred from one Shali'ach to another).

Question: What is the practical difference between Abaye's answer and Rava's?

Answer: They argue about Sheluchim asked to write a gift document;

Abaye holds like Rav, who says this is unlike asking them to write and give a Get. (The giver is not expected to write the document himself. There is no shame if someone else writes it, so they may make another Shali'ach.)

Rava holds like Shmuel, who says it is (Mili,) like asking to write a Get.

66b (Shmuel): If a man asked two people to write and give a Get to his wife, and they asked a scribe to write it, and they signed it, if she remarried relying on this Get, she must leave her new husband. This law requires investigation.

Even though Shmuel cited Rebbi to say that the Halachah follows R. Yosi, that Mili Lo Mimseran l'Shali'ach, Shmuel is unsure if the husband meant that they must sign it, or that they must write the Get itself.

(Mishnah): A Get written by a scribe and signed by one witness is valid;

(R. Yirmeyah): The text of the Mishnah says 'a Get signed by the scribe and one witness is valid.'

(Rav Chisda): The Mishnah is like R. Yosi, who says that Mili Lo Mimseran l'Shali'ach. (Chachamim say that Mili can be transferred. They forbid a scribe to sign, lest a man tell people to ask a scribe to write a Get, and to ask two others to sign it, and to avoid shaming the scribe (that he was not asked to sign it), they will make him a witness. The husband did not authorize this (so the Get is Pasul)!

It follows that R. Yosi does not allow transferring Shelichus even in a case of Omer Imru (telling someone 'appoint a Shali'ach for me'). Shmuel holds this works, but agrees with R. Yosi that Stam Mili does not.

Rishonim

Rif and Rosh (32b and 7:19): The Halachah follows R. Yosi, that Mili Lo Mimseran l'Shali'ach. Rav holds that if one told witnesses to write a gift document, they can tell another to write it. Shmuel says that he cannot, for it is Mili. The Halachah follows Shmuel in monetary laws.

Ran (DH v'Kaima): The Ramban says that a gift is like a Get only when he told three 'give'. Omer Imru is Pasul for a Get for it is not Lishmah, but it works for a gift. I hold like the Ro'oh, that if he appoints the Sheluchim it helps even for a Get, but Omer Imru does not work even for a gift. The Rambam agrees.

Ramban (cited in Ran DH v'Hiksheh): Omer Imru is not Mili. One can make a Shali'ach in the Shali'ach's absence! R. Yosi holds that a Get is like only if the scribe and witnesses hear from the husband himself. A woman can appoint a particular Shali'ach overseas to receive her Get. However, a Shali'ach Kabalah cannot make another Shali'ach, for this is Mili. A gift is like a Get regarding Mili, but there is no reason why Omer Imru would not help for a gift.

Rebuttal (Ro'oh, cited in Ran ibid.): R. Yosi disqualifies Omer Imru only when the Meshale'ach does not specify whom to appoint.

Question (Hasagos ha'Ra'avad DH Al): How could Omer Imru help? This is Ed mi'Pi Ed! The witnesses may not sign and the scribe cannot write Lishmah without hearing from the husband! Granted, R. Meir holds that the husband made a Beis Din (and the scribe and witnesses may rely on Beis Din). However, two are only witnesses! Perhaps the opinion that R. Yosi agrees that Omer Imru helps means that it helps if he tells three. This is even more difficult regarding a gift. How can they testify that the giver said so? Why didn't the Gemara ask this? We must say that Ed mi'Pi Ed applies only when the testimony is finished through their words. We are not concerned for it in documents, in which the testimony is finished when the owner or his Shali'ach gives the document.

Rosh (20): Only regarding writing a Get, the Shali'ach must hear from the husband. If it would be beneficial for a woman to be divorced, one could be Zocheh (receive a Get on behalf of her) without any appointment (Gitin 11b, Yevamos 118b). It seems that the same applies to a Shali'ach to give a Get.

Korban Nesan'el (80): The Gemara (28b) explicitly says that one can tell a Shali'ach to appoint someone else to be a Shali'ach to give a Get!

Rambam (Hilchos Zechiyah 4:10): A gift is like a Get; one cannot verbally command a (Shali'ach to make a second) Shali'ach. If one asked three people to tell Ploni and Almoni to write, sign and give a gift document to Levi, it is invalid. If they told Ploni and Almoni and they wrote and gave to Levi, he did not acquire. Similarly, if one asked two to write, sign and give a gift document to Levi, they cannot tell a scribe to write. They must write it, like a Get.

Magid Mishneh: Some say that if he said which Sheluchim to appoint, it is Kosher even regarding a Get, and surely also regarding a gift.

Poskim

Shulchan Aruch (CM 244:1): A gift is like a Get; one cannot relay words to a Shali'ach. Telling three 'tell Ploni and Almoni to write, sign and give a gift document to Levi' does nothing. If they told Ploni and Almoni and they wrote and gave to Levi, he did not acquire (Rema - and all the more so, if the three people themselves signed). Similarly, if one asked two to write, sign and give a gift document to Levi, they cannot tell a scribe to write. They must write it.

SMA (3): Regarding a gift, we are not concerned if the scribe is a witness. This is why the Rambam and Shulchan Aruch said that a gift is like a Get (only) regarding relaying words to a Shali'ach.

Shulchan Aruch (ibid.): Some say that it is Kosher if he said 'tell Ploni and Almoni to write and sign a gift document and give it to Levi.'

Tur (EH 141): The Ramah says that if a Shali'ach to receive a Get was authorized to make another Shali'ach, it does not help, for Mili Lo Mimseran. I disagree. Mili Lo Mimseran only when the Shali'ach was asked to do so himself. If he was told to tell others he can. We say later that if two were told 'tell Ploni to write and Levi and David to sign', this is not Mili.

Bedek ha'Bayis: This is difficult. The Tur himself (EH 120) said that in such a case, the scribe and witnesses may not write or sign until they hear from the husband! He does not discuss this later! The Radach says that 'later' refers to Choshen Mishpat 244. This is unreasonable. 'Later' means here, in Even ha'Ezer! Rather, the text should say 'like it says in the Gemara (66b).'

Rebuttal (Darchei Moshe 141:35): The Tur teaches that Omer Imru is not Pasul due to Mili. It is Pasul lest the scribe be improperly asked to sign.

Keneses ha'Gedolah (in Chidushei Hagahos 14): The Tur disqualifies Omer Imru due to Lishmah. The scribe and witnesses must hear from the husband.

Drishah (244:2 DH u'Mah she'Chosav): Here, the Tur says that Omer Imru works only for a gift. In Even ha'Ezer, he says that it works also for a Get! He means that it is not a problem of Mili, but it is Pasul for other reasons.

Rebuttal (Taz DH Imru): One opinion disqualifies Omer Imru for a Get, due to Mili. The Gemara said that we are not concerned for disaster, since the scribe may not sign without hearing the husband command him! We are stringent about a Get; and lenient about money.