Gemara

Question: May Reuven tell a Nochri 'muzzle my cow and thresh with it'?

We forbid Amirah l'Nochri (telling him to do Melachah) on Shabbos. Is this a stringency of Shabbos (since one who desecrates Shabbos is liable to death)? Or, does it apply even to Lavin, such as muzzling?

Answer - Question (from Eretz Yisrael): If Nochrim steal bulls from Yisraelim (their friends), castrate them and return them, what is the law?

Answer (Shmuel's father): The owners may not benefit from this ruse. They must sell them. (Castration is a Lav, yet one may not ask a Nochri to do it!)

Rejection (Rav Papa): In Eretz Yisrael, they hold like Rav Chidka, that Nochrim are forbidden to castrate. A Yisrael may not tell them to do so, due to "Lifnei Iver."

(Rava): Shmuel's father ruled that the bulls must be sold for slaughter.

Rejection (Abaye): It suffices that we fine the owner by forcing him to sell them.

Question: May one sell it to his minor son?

Answer #1 (Rav Achi): No.

Answer #2 (Rav Ashi): Yes.

Mereimar and Mar Zutra would exchange their bulls with each other.

Rishonim

The Rif (52b) brings our Sugya 90a.

Nimukei Yosef (DH Machlefei): All the Acharonim forbid Amirah l'Nochri even for a Lav, for all the Amora'im forbade castration through Nochrim. It is not due to Lifnei Iver, like Rav Chidka, for the Halachah does not follow him. A Yisrael may not tell a Nochri 'muzzle my cow and thresh your grain with it', or 'muzzle your cow and thresh my grain with it', for Isur is done with a Yisrael's property. The Machilta permits when it does not involve a Yisrael's property.

Rambam (Hilchos Kil'ayim 1:3): One may tell a Nochri to plant Kil'ai Zera'im for him.

Rebuttal (Ra'avad): In Eretz Yisrael, one must uproot Kil'ayim from his field! Also, the question of Amirah l'Nochri was not settled; we must be stringent.

Kesef Mishneh, citing Ri Korkus: The Rambam permits telling the Nochri to plant in the Nochri's field, even through Shelichus of the Yisrael and for his need. The Gemara asked about Amirah l'Nochri for the sake of a Yisrael. Even in the Yisrael's field, there is no Isur until it takes root.

Rambam (Hilchos Sechirus 13:3): a Yisrael may not tell a Nochri 'muzzle my cow and thresh with it. He is not lashed for this.

Magid Mishneh: We are stringent, for the question was not settled. This is the primary text of the Rambam. Some texts permit telling a Nochri to do so. We are lenient, for the question was not settled and it is a Safek mid'Rabanan.

Question (Mishneh l'Melech Hilchos Sechirus 3:13): We should be lenient about a Safek mid'Rabanan. Why is the Semag (and Ra'avad) stringent?

Rosh (7:6): The Gemara asked about Amirah l'Nochri for Lavim. This is unlike telling a Nochri to eat a Neveilah. A Yisrael may put Neveilah in a Nochri's mouth, just like he commands the Nochri! A Yisrael may not muzzle, so he cannot tell a Nochri to do so. Mereimar and Mar Zutra did not want Nochrim to castrate their bulls. They were stringent, for the Nochri wanted to help a Yisrael. The Ra'avad is lenient, for the question was not settled. I say that it was settled. All Amora'im except for Rav Papa forbid, even though the Halachah is unlike Rav Chidka.

Gra (EH 5:32): In Hilchos Kil'ayim, the Ra'avad says we are stringent, for the question was not settled. This requires investigation.

Hagahos Ashri: Since a Nochri is not commanded about muzzling, David may tell a Nochri 'muzzle my cow and thresh your grain with it', but not to thresh David's grain. One may not tell a Nochri to do something forbidden to him, due to Lifnei Iver. If we are sure that the Nochri himself decided to do something, even if it is to benefit his Yisrael friend, we do not fine the Yisrael and force him to sell it. We fine regarding castration, lest people scheme. Since the Nochri knows the Yisrael, stole the animal, castrated it and returned it, there are grounds to suspect that the Yisrael told him to do so.

Poskim

Shulchan Aruch (EH 5:14): One may not tell a Nochri to castrate a Yisrael's animal. If he took it himself and castrated it, it is permitted. If the Yisrael schemed (Rema - even if he did not scheme, but the Nochri knows him and intends to help him), we fine him. He sells it to another Yisrael.

Chelkas Mechokek (EH 5:8): The Ra'avad is lenient because Rav Papa, who was Basra, answered that Shmuel's father forbade due to Lifnei Iver. He rules like R. Chidka, and there is no source to forbid Amirah l'Nochri. In Choshen Mishpat (338:6), the Shulchan Aruch forbids Amirah l'Nochri.

Beis Shmuel (EH 5:16): One should be stringent like those who rule like R. Chidka (and not sell a rooster to a Nochri, for he will castrate it), and also like those who rule unlike R. Chidka (and forbid Amirah l'Nochri for all Isurim).

Shulchan Aruch (YD 296:2): Levi may tell a Nochri to plant Kil'ai Zera'im for Levi.

Taz (1) and Gra (EH 5:33): Why is this different than castration, which one may not tell a Nochri to do? It seems that we permit only in the Nochri's field.

Gra (7): We forbid only in the Yisrael's field, just like muzzling is forbidden only with the Yisrael's cow, even if it is for the sake of the Yisrael.

Shulchan Aruch (297 (the latter of this double Siman):4): One may not give an animal to a Nochri to mate it with another species.

Drishah (1): The Beis Yosef says that we are stringent about Amirah l'Nochri. I say that the Gemara did not ask about grafting or crossbreeding animals, for it forbids these to Nochrim, like the Rambam in Hilchos Melachim.

Mishneh l'Melech (Kil'ayim 1:6) and Dagul me'Revavah: The Kesef Mishneh (Kil'ayim 1:6) and Shach (3) say that the Rambam does not forbid these to Nochrim. They overlooked Melachim 10:6

Rema: Every Amirah l'Nochri, even for an Isur Lav, is forbidden, unless the Nochri acts for his own benefit.

Shach (4): This implies that it is permitted when the Nochri acts for his benefit, like Hagahos Ashri and the Magid Mishneh. Rashi, the Rosh and the Tur hold that the question was even in this case, and we are stringent. The Rosh says that one may tell a Nochri to do anything the Yisrael could do, e.g. feed Neveilah to the Nochri. If so, he may not tell him to muzzle and thresh the Nochri's grain. The Nimukei Yosef forbids with a Yisrael's cow, even if the Yisrael does not benefit.

Mordechai (Shabbos 459): One may not tell a Nochri to muzzle the Yisrael's cow and thresh the Nochri's grain, for this benefits the Yisrael: he can ask for more rent, since he allows muzzling.