Gemara

(Beraisa): If Reuven put an Eruv on a monkey or elephant and he told Shimon to receive the Eruv from it, the Eruv is valid.

Question: Perhaps the animal did not bring it!

Answer (Rav Chisda): The case is, Reuven saw the animal bring it.

Question: Perhaps Shimon did not take it from the animal!

Answer (Rav Yechi'el): Chazakah Shali'ach Oseh Shelichuso (there is a Chazakah that a Shali'ach fulfils his mission).

(Rav Nachman): We rely on the Chazakah only for mid'Rabanan laws.

Rav Sheshes): It is even for Torah laws.

Support (Rav Sheshes, for himself - Beraisa): If Levi (a Chaver, who is trustworthy about Ma'aser) told David 'go fill this basket with figs from my tree', David may eat without tithing. (Surely, Levi took Terumah);

i. R. Shimon ben Gamliel disagrees for a side reason, but all agree that Chazakah Shali'ach Oseh Shelichuso! (Levi is like a Shali'ach to separate for David.)

Rejection (Rav Nachman): There is different, for Rav Chanina Chuza'ah taught that there is a Chazakah that a Chaver does not allow Tevel to leave his Reshus.

Chulin 12a - Questions (Rav Dimi bar Yosef): If Reuven asked a Shali'ach to slaughter for him and later found the animal slaughtered, or to tithe for him and later found the tithes separated, what is the law?

Answer (Rav Nachman): He may assume that the animal was slaughtered properly, but not that the tithes were separated properly.

Objection (Rav Dimi bar Yosef): This is inconsistent!

If Chazakah Shali'ach Oseh Shelichuso, we should rely on also for tithes. If there is no Chazakah, why may we rely on the Shechitah?

Answer (Rav Nachman): Really, there is no Chazakah;

Regarding Shechitah, even if Ploni overheard Reuven command the Shali'ach (and Ploni slaughtered the animal), most who slaughter know the laws, so we assume that Ploni is from the majority;

Regarding tithes, if Ploni overheard and tithed, it is invalid, for one cannot take Ma'aser without the owner's request.

Gitin 64a (R. Yochanan): If Reuven made a Shali'ach to give a Get, and the Shali'ach says that he gave it, and she says that she received it and lost it, she needs two witnesses to remarry.

Question: Why can't we rely on Chazakah Shali'ach Oseh Shelichuso?

(R. Yitzchak): If Levi told Shimon 'be Mekadesh a woman to me', and Shimon went away and died, Levi may not marry any woman, due to Chazakah Shali'ach Oseh Shelichuso. (Perhaps Shimon was Mekadesh to Levi the sister of whomever Levi wants to marry now!)

Answer: We rely on the Chazakah only to be stringent, but not to be lenient.

Rishonim

Rambam (Hilchos Eruvin 6:22): If Reuven told Shimon to receive an Eruv from a Cheresh, lunatic, child, or even an animal, and Reuven saw that Shimon received it, it is a valid Eruv. Chazakah Shali'ach Oseh Shelichuso. Many who joined to make Eruv Techumim may send it with a Shali'ach.

Rosh (3:3): R. Shimshon ha'Zaken rules like Rav Sheshes, for Ge'onim say that the Halachah follows him in Isurim [when he argues with Rav Nachman]. R. Tam rules like Rav Nachman. He brought a proof from Gitin 64a, which says that the Chazakah is only to be stringent. Another proof is from Chulin 12a, in which we say that all agree that there is no Chazakah. Even though there Rav Nachman said so, since the Sugya concludes like him, and the questioner accepted his answer, this shows that the Halachah follows him.

Rebuttal (Rosh): There is no proof from Gitin and Chulin. Rav Sheshes says Chazakah Shali'ach Oseh Shelichuso only when the Shali'ach must be concerned for Michshol (if he does not fulfill his mission, the Meshale'ach will rely that he fulfilled it, and transgress). Therefore, the Shali'ach is not concerned if he does not give the Get. He is not concerned if he does not tithe or slaughter, for no transgression will result. The Meshale'ach will know that [the animal was not slaughtered, and] that the pile was not tithed if he does not see Terumah near the pile. Since R. Tam's proofs are rejected, we need not contradict the Ge'onim's rule.

Note: The Ri (in Tosfos 32a DH Rav) rejected R. Tam's proofs like the Rosh did. The Shali'ach does not consider that someone else might hear the command and tithe.

Tosfos (64a DH Asur): Why don't we forbid every minor orphan, lest her father was Mekadesh her through the Shali'ach? It is only a fine to forbid the Meshale'ach, who was not concerned for Takalah. Letter of the law, he may marry any woman. It is not a proper Chazakah, for perhaps she will not accept Kidushin. Also, we rely on the majority of the world. (Most women are permitted to him.) If not for Chazakah Shali'ach Oseh Shelichuso, we would not make such a fine.

Maharam me'Rotenburg (380): R. Shimshon rules like Rav Sheshes, that Chazakah Shali'ach Oseh Shelichuso is even for a Torah Isur. Seder Tana'im and Amora'im says that the Halachah follows him against Rav Nachman in Isurim. This is only when there would be Takalah if he does not fulfill his mission.

Poskim

Shulchan Aruch (OC 409:8): If Reuven told Shimon to receive an Eruv from an animal, and Reuven saw that Shimon received it, it is a valid Eruv, even if he did not see Shimon place it. Chazakah Shali'ach Oseh Shelichuso. Many who joined to make Eruv Techumim may send it with a Shali'ach.

Magen Avraham (17): The Chazakah is even for an Isur Torah, e.g. when Michshol applies, like here. If we know that the Shali'ach did the Shelichus, just we are unsure whether he deviated, we say that surely, he did not deviate.

Mishnah Berurah (49): This is only if the Shali'ach explicitly promised to receive the Eruv and place it for him. If he was silent, we do not say Chazakah Shali'ach Oseh Shelichuso.

Mishnah Berurah (50): We rely on the Chazakah only for an Isur mid'Rabanan. For an Isur Torah, we are only stringent. Some say that even for an Isur Torah, if Michshol applies, like here, we rely on the Chazakah.

Bi'ur Halachah (DH v'Nasnu): The Mechaber cites the Tur, who requires that he see Shimon receive it. The other Poskim (Rambam, Rashi, R. Yehonason, Ri'az and Avodas ha'Kodesh) hold that the Chazakah suffices. The Gemara asked that perhaps he never received it, and answered that Chazakah Shali'ach Oseh Shelichuso. This shows that the Chazakah shows that he received it. The Tur is difficult. He himself wrote like the other Poskim in his Kitzur Piskei ha'Rosh.

Kaf ha'Chayim (64): The Gemara and Rambam connote that it suffices to see that the animal brought it, but the Levush and Chayei Adam rule like the Shulchan Aruch, who requires that he see the Shali'ach receive it.

Mishneh l'Melech (Hilchos Bechoros 4:1): A man made a Shali'ach to sell part of his animal to a Nochri to exempt it from Bechorah. For Torah laws, we rely on Chazakah Shali'ach Oseh Shelichuso only to be stringent, like it says in Gitin. However, sometimes we rely on it to be lenient.

Suggestion: The Chazakah applies for Torah laws only if he knows that the action was done.

Rejection (Mishneh l'Melech): Only Rav Nachman distinguishes. Rav Sheshes equates Torah and mid'Rabanan laws. How can the Tur (YD 331) say that we rely on the Chazakah for Ma'aser? The Beis Yosef brought the Sugya, and did not mention that it says just the contrary in Chulin! The Chazakah does not apply to something that depends on others. Perhaps no Nochri will want to buy! Tosfos (Gitin 64a) says that it is a mere fine to forbid the Meshale'ach to all women. Since perhaps no woman will accept Kidushin, the Chazakah does not apply even to be stringent!

Mishneh l'Melech (DH Ho'il and DH u'Milvad): The Rif, Rambam, R. Tam and Rashba do not rely on the Chazakah to be lenient for a Torah Isur. We must be concerned for their opinion. The Chazakah applies only to something without a source in Torah, like Techumim. Rav Sheshes brought a proof from Terumah of figs, which is only mid'Rabanan. Even though we are lenient about a Safek mid'Rabanan even without a Chazakah, the Chazakah permits making a Shali'ach l'Chatchilah. One may not create a Safek mid'Rabanan l'Chatchilah and be lenient about it.