Gemara

Perhaps the Isur to do business with Nochrim (before their festivals) is lest a Yisrael sell something that the Nochri will offer to idolatry, and the Yisrael transgresses "v'Lifnei Iver Lo Siten Michshol" (by enabling him to sin). If the Nochri already has an animal, 'Lifnei Iver' does not apply.

Question: 'Lifnei Iver' applies even when he has his own animal!

(Beraisa - R. Noson): "V'Lifnei Iver Lo Siten Michshol" forbids extending wine to a Nazir or a limb from a living animal to a Nochri.

Even though he could take it himself, one who gives to him transgresses!

Answer: The case is, he is on the other side of a river, and he cannot reach it himself.

Support: The Beraisa mentions extending it to him, and not 'giving' it.

15b: Rabah sold a donkey to a Yisrael suspected of selling to Nochrim. Abaye questioned this, for perhaps the buyer will sell to a Nochri!

Rabah: He might, but also he might sell to a Yisrael. Therefore, I may assume the latter.

Nedarim 62b: Rav Ashi sold a forest to priests who served fire.

Question (Ravina): You transgress "v'Lifnei Iver"!

Answer (Rav Ashi): Most wood is burned (not for idolatry).

Rishonim

Rif (1b): Lifnei Iver applies only when the sinner could not do so himself.

Rambam (Hilchos Shemitah 8:8): We encourage Nochrim working in Shemitah, for they are not commanded to let the land rest.

Rosh (1:2): One may not give Isur to a Mumar, even if it is the Mumar's, if he is on the other side of the river.

Rosh (15): Rav Ashi sold a forest to priests of idolatry. It was not Lifnei Iver because most wood is burned not for idolatry. If not for this, it would be forbidden. Also Re'em explained like this.

Rosh and Ran (Shabbos 1:1 and 1a DH u'Makshu): If Ploni could take an Isur by himself, and David gave it to him, David transgresses Lifnei Iver mid'Rabanan. Even when a minor eats Neveilah, Beis Din must stop him, all the more so one may not help an adult to sin!

Ran (Avodah Zarah 1a DH Lifnei): Some say that the Isur to do business with Nochrim before their festival is due to Lifnei Iver, lest they buy something and offer it to idolatry. If the Nochri already has his own animal that he could offer, Lifnei Iver does not apply.

Ran (1b DH Deika): The Gemara connotes that there is no Isur Torah (to give an Isur to one who could take it himself), but in any case it is forbidden mid'Rabanan. One is commanded to stop others from transgressing. How can one help them transgress?!

Ran (7a DH Garsinan, citing the Ramban): Lifnei Iver, even mid'Rabanan, applies only to what he could not do without the Yisrael, e.g. giving wine to a Nazir.

Mordechai (795): (Lifnei Iver does not apply when the Nochri could take it himself). Bahag learns from here that nowadays we may lend to Nochrim, even though they use the money to give to the priest, for they give only a small amount that they could borrow from other Nochrim even if we did not lend to them. Avi ha'Ezri permits selling to a Nochri priests' garments in which they sing to idolatry as long as the priests have other garments, even though they surely buy for idolatry.

Poskim

Rema (YD 151:1): Some say that the Isur to sell to Nochrim things for their Avodah is only if they do not have others like them, or they cannot buy them elsewhere, but if they can, one may sell them anything. Some are stringent. The custom is to be lenient like the first opinion. A Ba'al Nefesh should be stringent for himself.

Shach (6): The Rema holds that the lenient opinion equates this to passing wine to a Nazir. It is permitted if he could take it himself. The stringent opinion says that even then it is forbidden mid'Rabanan. I say that all agree with the Mordechai and Tosfos who permit regarding a Nochri or Mumar. The Rishonim who forbid discuss a Yisrael. One must separate a Yisrael from Isur, even a child, and all the more so an adult. If we will not say so, the Rosh and R. Yerucham contradict themselves (for in one place they forbid, and elsewhere they permit).

Gra (8): Tosfos (6b DH Minayin) says that the Isur is only if he cannot get it himself. The Ran, and Tosfos and the Rosh in Shabbos say that in any case it is forbidden mid'Rabanan. The Gemara in Avodah Zarah connotes that it is permitted even mid'Rabanan. The Isur (of business with Nochrim before their festivals) in the Mishnah (2a), which we suggest is due to Lifnei Iver, is only mid'Rabanan. Therefore, when we say that there is no Lifnei Iver when the Nochri already has what to offer, this must mean even mid'Rabanan it is permitted. To answer the Ran's question, we can say that helping transgressors is forbidden only regarding Yisrael. The Mordechai's law is wrong. We say that one does not transgress Lifnei Iver only when he already has the item, but not just because he could get it elsewhere. If not, why is it forbidden to sell wood to priests? There is wood elsewhere! Or, we could say that there is Lifnei Iver mid'Rabanan even when the Nochri already has what to offer. The Mishnah is only mid'Rabanan because we do not know that he buys for idolatry. In such a case it is permitted if the Nochri has his own. The Gemara in Nedarim shows that the latter opinion is correct. It is very difficult to say that the entire Mishnah, and the case of Rabah who sold a donkey, all discuss when the Nochri could not buy them elsewhere.

Birkei Yosef (1): Nedarim 62 shows that mid'Rabanan one may not give to a Nochri (even if he could get himself). Surely anyone can buy wood, and even so Ravina thought to forbid! How can the Mordechai answer? It is difficult to say that other wood was not available. The Rambam says that we encourage Nochrim working in Shemitah only because they are permitted. If they were forbidden, it would be Asur to encourage them even verbally! The Ran (1b) forbids mid'Rabanan when the person could take it himself. It seems that he refers to both giving wine to a Nazir or Ever Min ha'Chai to a Nochri. However, Piskei Tosfos (Shabbos 1) says that one must separate Chavero (a Yisrael) from an Aveirah. This is like the Shach.

Emunas Shmuel (14, cited in Pischei Teshuvah 2): The Mordechai's Heter is not clear. The Gemara said that if business is Asur due to Lifnei Iver, it is permitted when the Nochri has an animal. It did not permit when he could buy one! The Mordechai's Heter would annul the Isur of business. Surely Nochrim can buy animals and frankincense elsewhere!

Pischei Teshuvah: Perhaps we can answer based on what I wrote below (160:1) from the Mishneh l'Melech (Hilchos Malveh 4:2. If a Nazir could take wine by himself, if no one gives it to him, even if he takes it, no Isur of Lifnei Iver is done. If one wants to borrow from a Yisrael with Ribis (interest), and sees that the lender is ready to lend to another Yisrael with Ribis, in either case there is Lifnei Iver, so the borrower transgresses Lifnei Iver even though the sinner would have sinned even without him.)

(Note: Pischei Teshuvah's answer is that here, the Nochri could buy from another Nochri, who is not commanded about Lifnei Iver, so even a Yisrael who sells to him does not transgress Lifnei Iver. - PF)

He'aros on Birkei Yosef (1): The other Acharonim hold that Lifnei Iver does not apply even when the sinner could buy it elsewhere. We can say that the Nochri could not have bought another animal or frankincense that day, therefore, it is like two sides of the river.

Eshel Avraham (OC 163:2): Lifnei Iver is a Lav, and helping transgressor is mid'Rabanan. This is why the Gemara connotes that there is no decree regarding Nochrim.

Mishnah Berurah (347:7): Most Poskim say that helping transgressors does not apply to Nochrim, unless the Nochri could not have done so himself, for then Lifnei Iver applies. The Gra forbids even when the Nochri could get the Isur elsewhere.

Pri Megadim (Petichah Koleles 5:8): If Lifnei d'Lifnei (causing one to cause another to sin) is mid'Oraisa, how can we distinguish Yisrael from Nochrim? This requires investigation.

Note: Only Yisrael are commanded about Lifnei Iver, so Lifnei d'Lifnei is forbidden, for he causes the Yisrael to transgress Lifnei Iver!

See Chavos Ya'ir 185 (Shlichus Nochri l;chum)

Rema (EH 5:14): One may sell animals and roosters to a Nochri, even though the Nochri will castrate them. Some forbid.

Beis Shmuel (17): Normally, Lifnei Iver does not apply if the receiver could buy the Isur elsewhere. That is when he is limited (how much he will drink or offer to idolatry). Here, he will castrate all animals he gets!