Gemara

6a (Mishnah): One who gives to his mother-in-law (dough to bake) must Ma'aser the dough he gives her and the bread he receives, because she will switch it if it is spoiling.

7b: R. Pinchas ben Ya'ir saw white mules in Rebbi's premises, and said 'I cannot eat with one who keeps the angel of death (damagers) in his home!'

Rebbi: I will sell them!

R. Pinchas ben Ya'ir: "V'Lifnei Iver Lo Siten Michshol (do not put a stumbling block before the blind"! Whoever buys from you will transgress.)

Avodah Zarah 6a: If a Yisrael sells an animal to a Nochri shortly before his festival (perhaps he will offer it to idolatry), if the Nochri already has an animal, 'Lifnei Iver' does not apply.

Question (Beraisa - R. Noson): Lifnei Iver forbids extending wine to a Nazir or a limb from a living animal to a Nochri.

Even though the Nazir or Nochri could take it himself, one who gives to him transgresses!

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

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

21b (Beraisa - R. Shimon ben Elazar): One may not rent his field to a Kusi, for people call it the Yisrael's, and the Kusi will work on Chol ha'Mo'ed.

Question: In any case it should be forbidden due to "Lifnei Iver"!

Answer: Indeed, the Tana forbids due to Lifnei Iver, and also because people call it the Yisrael's field.

Bava Metzi'a 75b (Mishnah): The lender, borrower, guarantor and the witnesses (on a loan on interest) transgress Lifnei Iver.

Rishonim

Rif (Avodah Zarah 1b): Lifnei Iver is forbidden only when the sinner could not do so himself. This is why the Gemara forbids to extend wine to the Nazir, for he is on the other side of the river and he could not take it himself.

Rosh (Avodah Zarah 1:2): The Gemara discussed giving Ever Min ha'Chai to a Nochri to teach that Lifnei Iver applies to even to Nochrim. It discussed giving wine to a Nazir because a Nazir is not careful not to drink it, for he drank all his life and he sees others drinking. Therefore, even if he asks Stam one may not give to him, lest he forget his Nezirus and drink. Therefore, 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.

Mordechai (Bava Metzia 247): The Ri says that even though one may lend to a Mumar with Ribis, one may not borrow from him with Ribis. The Mishnah permits to borrow from a Ger Toshav with Ribis. Since it does not permit borrowing from a Mumar with Ribis, we infer that it is forbidden, due to Lifnei Iver.

Question (Gra YD 159:3): Sefer ha'Terumos (346) brings from Maseches Kusim that one may also lend to or borrow [from Kusim] with Ribis, and learns from this that they are real converts.

Mordechai (Avodah Zarah 795): Bahag forbids to give Yayin Nesech to a Mumar. Even though he sinned, he is a Yisrael. This is only if they are on opposite sides of a river. This shows that nowadays one may lend to Mumarim with Ribis. Even though they give some of the money to the munk to offer on their festival, they give a small amount. They could borrow it from other Nochrim, even if we would not lend to them. Also Avi ha'Ezri permits selling to them priestly garment in which they sing to idolatry, whenever there are other priests, even though they surely bought for the sake of idolatry.

Tosfos (Avodah Zarah 15b DH l'Oved): The Gemara concludes that one may not sell (animals) to Yisre'elim suspected to sell to Nochrim. Above, we permit Lfnei d'Lifnei (causing another to transgress Lifnei Iver). That refers to causing a Nochri cause another to sin, since Nochrim are not commanded about Lifnei Iver. We are commanded that no Yisrael come to transgress through us. R. Elchanan said so. Also, selling to a Yisrael is (one) Lifnei Iver, lest the buyer lend or rent his animal to Nochrim (it will work on Shabbos) or make it work on Shabbos (if a Nochri buys it just before Shabbos, and complains that it refuses to work).

Tosfos (Avodah Zarah 22a DH Teipuk): R. Tam says that Lifnei Iver applies even to Isurim mid'Rabanan. He forbids Melachah on Chol ha'Mo'ed only mid'Rabanan (and Lifnei Iver applies to causing Kusim to work on Chol ha'Mo'ed). Another proof is from the Isur to sell to a Yisrael suspected of selling (animals) to Nochrim, even though the Isur (to sell animals to Nochrim) is only mid'Rabanan.

Chavos Ya'ir (185): I say that the seller transgresses Lifnei Iver, for any Aveirah that the buyer is commanded about and transgresses (with what he bought).

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 first opinion equates this to passing wine to a Nazir. It is permitted if he could take it himself. The stringent opinion forbids even this mid'Rabanan. I say that they do not argue. All agree to the Mordechai and Tosfos in Avodah Zarah, who permit regarding a Nochri or Mumar. The Mordechai, Tosfos and Rosh in Shabbos, and the Ran in Avodah Zarah, forbid. They discuss a Yisrael, for we must separate him from Aveiros. Beis Din must separate even a minor eating Isur, and all the more so an adult! The Rosh in Avodah Zarah says like Tosfos in Avodah Zarah. If we do not say like I said, the Rosh, and also R. Yerucham, contradict themselves.

Pischei Teshuvah (4): Chulin 6a supports the Shach's distinction (that we must separate a Yisrael from transgression).

Question: Reuven was a partner with a Nochri (Tony) in a tannery. For years, he would give the rump and Tereifos to Tony, and deduct their value from Tony's share of the business. Tony married a Mumeres, and fathered children from her. If Reuven continues to give meat to Tony, the Mumeres will eat Chelev, blood and Tereifah. In any case Tony can buy meat for her elsewhere. The Gemara forbids giving wine to a Nazir only when could not take it himself!

Answer (Chavos Ya'ir (185): Tosfos and the Rema say so. However, Mordechai and the Ran are stringent. The Rema wrote that the custom is to be lenient. This is why people sell Sha'atnez and Treifos to Mumrim. Indeed, we don't protest against them, but we do not counsel them to do so, like the Gemara. (It exempts one who gave wine to a Nazir who could get it himself. It did not permit - PF.) Especially, here Reuven gives to Tony without immediate payment. Others would not sell to him like this! Whenever we permit because the sinner could get it elsewhere, this is only when he could get it easily without toil. Therefore, even if one may sell a Beged of Sha'atnez, this refers to Golem (plain cloth), but not a garment. (Note: i.e. he bought the fabric from a big manufacturer who sells to merchants. If not, perhaps none of the merchants' cloth is Sha'atnez - PF). Surely, if merchants are available in the city, the lenient opinion permits, but not if he would have to toil to go afar to get it. One may not sell a finished garment of Sha'atnez, even if many Nochrim sell in the city, for it is hard to find one that fits his size for the proper price.

Chavos Ya'ir: I learn from Avodah Zarah. If one sells to a Nochri who already has an animal, 'Lifnei Iver' does not apply. This implies that if he does not have an animal, "Lifnei Iver" applies, even though he could buy from others. This is because it is a toil to buy. Also, we say that the borrower transgresses Lifnei Iver, even though the lender could lend (to Yisre'elim - PF) with Ribis through Nochrim, for many Nochrim do so (and also in such a case, the lender transgresses - PF), because it is a toil for the lender to do so. Therefore, one may not sell Yayin Nesech to a Mumar, for it is not clear that he could get good wine for this price. Tosfos proved that Lifnei Iver applies even to Isurim mid'Rabanan. The Mordechai (Avodah Zarah 795) connotes otherwise (that one may give Yayin Nesech to a Mumar if they are on the same side of the river). This is difficult (for one may give to him only if he could get it himself without toil.) All the more so one may not give to him for free! Likewise, one may not be a middleman to find a harlot for a Yisrael who desires this, for it is not easily available to the Yisrael. Some are lenient to make matches among Nochrim, even though this leads to birth of idolaters. One reason is that nowadays, Nochrim do not really serve idolatry. Even though they consider Hash-m to have three 'parts', they are not commanded about Hash-m's unity. This is not a reason to be lenient about a Mumar.

Suggestion: He may give to Tony's slave to give to Tony, for this is like Lifnei d'Lifnei. (One may do so to a Nochri, who is not commanded about Lifnei Iver.)

Rejection (Chavos Ya'ir): Since he sends her to give to him, it is as if he gives to him. We hold that (mid'Rabanan) Shelichus applies to Nochrim when this is a stringency. Even so, I permitted (selling to Tony) for a few reasons. 1. Those who forbid (when the sinner could get it himself) say so only about a Yisrael, for one must separate a Yisrael from sinning. One need not separate a Nochri or Mumar from sinning. 2. Even those who forbid, forbid only mid'Rabanan. The Shach distinguishes a Yisrael from a Mumar; he permits regarding a Mumar. If Reuven insists that from now on, Tony must pay for the meat immediately, surely it is permitted. 3. The Rema said that some are stringent. He did not say that some forbid. This shows that he permits. 4. Since men rule in their homes, and Tony buys the meat, even though he gives to the Mumeres, this is Lifnei d'Lifnei. If Tony himself was a Mumar, it is forbidden even if he does not eat and gives all of it to her. 5. Since Reuven already sold to him for years, this is reason to be lenient - 'he entered my domain. I did not enter his.' 6. If he stops selling now, this will comes to strife. Often, Chachamim were concerned for this. Mumarim often make problems for Yisre'elim. In any case, a pious person should try to avoid such situations.

Note: Pischei Teshuvah (3) says that since the man rules in his house, one may sell even on credit. I did not see Chavos Ya'ir say so. However, 'if Reuven insists that from now on, Tony will pay immediately, surely it is permitted' connotes that it is not clear to forbid on credit.