Gemara

(Ravina): Ein Shali'ach l'Devar Aveirah only when the Shali'ach is Bar Chiyuva (liable for transgressing).

(Rav Sama): Ein Shali'ach l'Devar Aveirah only when the Shali'ach can choose whether or not to comply.

They argue about a Kohen who told a Yisrael 'be Mekadesh a divorcee to me', or a man who told a woman 'cut a boy's sideburns for me.' Rav Sama exempts the Meshale'ach, and Ravina obligates him.

Kidushin 42b - Question: Yesh Shali'ach l'Devar Aveirah for Me'ilah. Why don't we learn from Me'ilah to other Aveiros?

Answer #1: Me'ilah and Shlichus Yad are Shenei Kesuvim, therefore, we do not learn to other cases.

Answer #2: According to Beis Shamai (who disagree about Shelichus Yad), Me'ilah and selling or slaughtering a stolen animal are Shenei Kesuvim.

Answer #3: According to the opinion that that Shenei Kesuvim teach to other places, Beis Shamai learn from Shechutei Chutz (slaughtering Kodshim outside the Mikdash), it says "to that man", not to his Shali'ach. (An extra verse teaches that we learn to all other Aveiros!)

Gitin 40a (Rav Dimi): If one said before he died that his heirs should not make his Shifchah work, we force them to free her.

Objection (R. Ami and R. Asi): (If she is not freed,) her children are slaves. Why must the heirs free her? (It suffices that they not make her work!)

Sotah 44a (Beraisa): "He did not (yet) marry her" teaches that one who was Mekadesh a forbidden wife, e.g. a widow to a Kohen Gadol, fights in war.

Suggestion: This is unlike R. Yosi ha'Glili, who says that the softhearted person who returns is one who is afraid of his sins.

Rejection: It is even like R. Yosi ha'Glili. Rabah taught that a Kohen Gadol who was Mekadesh a widow is not liable (for lashes) until he has Bi'ah with her.

Rishonim

Rif and Rosh (Kidushin 16b and 2:2): Actions of David's Shali'ach are attributed to David, unless he was told to do an Aveirah. Then, we say that he should have listened to Hash-m, not to David.

Rambam (Hilchos Me'ilah 7:1): If Reuven told Shimon "give one piece of meat to each guest," and Shimon told the guests 'I allow you to take two each', and the guests took three each (and the meat was found to be Hekdesh), all were Mo'el.

Rambam (2): This refers to meat of Bedek ha'Bayis. If the meat was of an Olah, only the ones who ate transgressed Me'ilah, for there was another Isur in addition to Me'ilah. In the entire Torah, Ein Shali'ach l'Devar Aveirah, except for Me'ilah with no other Isur involved.

Question (Tosfos Bava Metzia 10b DH d'Omar): According to Rava, even if Yesh Shali'ach l'Devar Aveirah, a Kohen is not lashed for Kidushin, until Bi'ah!

Answers (Tosfos): If Yesh Shali'ach, after Bi'ah he is lashed also for the Kidushin. Alternatively, if Ein Shali'ach, the Kidushin is invalid.

Question (Maharsha (Kidushin 43a DH Gemara): Why was an extra verse needed to teach not to learn Yesh Shali'ach l'Devar Aveirah from Me'ilah and Shechitah? If we learned this, we would not need verses to teach about Shelichus for divorce and Kidushin!

Answer #1 (Maharsha): We would not learn Mitzvos from Aveiros. Perhaps it is a fine that Yesh Shali'ach l'Devar Aveirah.

Rebuttal (and Answer #2 - Noda bi'Yehudah EH 75): Surely, it is not a fine regarding Me'ilah. The Meshale'ach is liable without Shelichus! Rather, we asked that the Shali'ach should be liable, and the Meshale'ach should be exempt. Surely, Shelichus l'Devar Aveirah has no Chalos (the Kinyan or Kidushin does not take effect). We needed verses to teach that Shelichus for divorce and Kidushin is Chal. Ein Shali'ach l'Devar Aveirah connotes that it is void. Ran and Answer #2 in Tosfos say so. The Rambam must agree, for he (Hilchos Isurei Bi'ah 17:3) rules unlike the first answer. Answer #1 agrees that the Shelichus is not Chal. It holds that since there was no Aveirah in the Kidushin itself, it is Chal, even though Bi'ah forbids the Kidushin retroactively. We asked (Kidushin 42b) why we don't learn from divorce to obligate a Meshale'ach for an Aveirah. If we could distinguish Chiyuv from Chalos, perhaps we only learn from divorce that it is Chal, but there is no source to obligate the Meshale'ach! Rather, he is obligated if and only if his Shali'ach is considered like him. The Rambam says that Yesh Shali'ach l'Devar Aveirah only regarding Me'ilah, i.e. only there we need to say that it is Chal. Shechitah and Shelichus Yad (use of a deposit) do not require Shelichus.

Maharit: Beis Hillel use the Drashah to obligate for Shechitah or selling via one who cannot be a Shali'ach. Beis Shamai use it for Shenei Kesuvim, and obligate only via a Shali'ach. Perhaps we conclude that they obligate via anyone, and learn Ein Shali'ach l'Devar Aveirah from Shechutei Chutz.

Ha'Makneh (Kidushin 42b DH Divrei): According to Answer #2 in Tosfos, when Ein Shali'ach, the action (e.g. Kidushin) is not Chal. What is his source? Perhaps Ein Shali'ach merely exempts the Meshale'ach! It seems that he learns from the Tosefta that the Rambam brought. If the meat was an Olah, only the one who ate was Mo'el, for the only Shali'ach l'Devar Aveirah is Me'ilah with no other Isur involved. It seems that the other Aveirah cancels the Shelichus, even regarding Me'ilah. The Rambam holds that Me'ilah is a Chidush, for only there one benefits and another is lashed. If so, we cannot learn from Me'ilah that another Shelichus l'Devar Aveirah is Batel due to another Aveirah. Sotah 44a supports Tosfos. We thought that R. Yosi would disqualify one who was Mekadesh a forbidden wife, for the Kidushin was forbidden. If Ein Shali'ach does not Mevatel the Chalos, perhaps he was Mekadesh her through a Shali'ach! However, perhaps it is Chal; the Mekadesh does not fight because a Meshale'ach is liable b'Yedei Shamayim.

Ha'Makneh: The Rema (YD 160:16, citing Rashi) permits to tell Levi 'lend to me Moshe's money, with Ribis', because Ein Shali'ach. He must hold that the Shelichus is Batel. If not, the borrower pays Ribis to the lender! The Taz (11) rejects this, for a Madir (one who vowed not to benefit Ploni) may not send to Ploni via a Shali'ach. Ein Shali'ach exempts the Meshale'ach only from Kares or lashes, but it is forbidden. Rashi can say that Ein Shali'ach does not apply here, for the Isur (to benefit) applies only to Ploni (Ran Nedarim 15b). Even though Lifnei Iver applies, Tosfos says that we do not say Ein Shali'ach due to Lifnei Iver. However, the Rambam holds that the Madir is lashed. He holds that the Shelichus is Batel only regarding Me'ilah. A support is a Get to free an Eved Kena'ani. The master transgresses 'l'Olam Bahem Ta'avodu" (Gitin 38a), yet the Shelichus is Chal! However, the primary Isur is neglecting to make the slave work, and this the master does himself.

Note: What is the source that the primary Isur is not making the slave work? Heirs must free a Shifchah if their father commanded that they not make her work. Tosfos (38a DH Kol) explains that since they cannot make her work due to the Mitzvah to fulfill the deceased's request, there is no Isur to free her. Tosfos did not ask why it is a Mitzvah to fulfill this request! Many do not require Shelichus to write a Get (Tosfos Gitin 22b DH v'Ha). Signing requires Shelichus, but surely this is not Shelichus l'Devar Aveirah if Edei Mesirah are always required (Tosfos Gitin 4a DH d'Kaima) and they are primary.