GEMARA

6a (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 the Nazir or Nochri could take it himself, one who gives him transgresses!

Rejection: 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, 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.

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

26a (R. Avahu - Beraisa): We throw into a pit Minim (priests of idolatry)... and Mumrim (people who habitually transgress a certain Mitzvah).

Objection (R. Yochanan - Beraisa): "L'Chol Aveidas Achicha" includes the Aveidah (lost object) of a Mumar.

We must return his property. All the more so we do not kill him!

Question: Perhaps we must return the Aveidah of a Mumar who eats Neveilos l'Te'avon (to fulfill his appetite), and the Beraisa teaches that we kill a Mumar who eats Neveilos Lehach'is (to anger Hash-m)!

Answer: R. Yochanan holds that a Mumar Lehach'is is considered a Min (which was already mentioned in R. Avahu's Beraisa).

Bava Metzia 70b (Mishnah): One may borrow from or lend to a Nochri or Ger Toshav (a Nochri who accepted to observe his Mitzvos) with Ribis.

RISHONIM

Rambam (Hilchos Mamrim 3:3): (One who does not admit to oral Torah is an Apikores, and anyone may kill him.) This is if he followed his heart to deny based on his thinking to initiate denial of oral Torah, like Tzadok and Baisus and all who strayed after them. The descendants of the strayers who were enticed by their fathers and grew up among the Kara'im and raised with their creed, they are like a captured baby that was raised by them. It is as if he is Anus (forced). Even if he later heard (that he is a Yisrael and saw Yisre'elim and their creed), he is Anus because he was raised on their error. It is proper to return him in Teshuvah and draw him with peaceful words until he returns to Torah.

Perush ha'Mishnayos (Chulin 1:2): Tzedukim and Baisusim denied oral Torah. Nowadays they are called Stam Minim, i.e. even after the Churban one may kill them like Minim. This refers to those who began it, for this is the opening to true heresy. We kill the initiators, lest they lead Yisre'elim astray and ruin the faith. This was fulfilled in practice many times in the west. One who was born and raised according to this is like an Anus. His law is like a captured baby that was raised by Nochrim. All his Aveiros are Shogeg. The initiator is Mezid.

Rosh (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.

Rosh (2:7 and Bava Metzia 5:52): If a Mumar is Kofer b'Ikar (denied basic tenets of Torah), one may lend to him with Ribis (interest). The Yerushalmi permits this. He is not "your brother." One may not take Ribis from a Yisrael, due to "v'Chai Achicha Imach." There is no Mitzvah to help a Mumar to idolatry live. However, one may not pay Ribis to a Mumar. Even though he sinned, he is a Yisrael. The Isurim of Lifnei Iver and paying Ribis apply. He does not transgress Lifnei Iver when he pays Ribis to us, for we may lend to him with Ribis.

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 proper converts.

POSKIM

Rema (YD 151:1): Some permit selling to Nochrim things for their Avodah if they have others like them. Some forbid.

Shach (6): All agree with the Mordechai and Tosfos who permit regarding a Nochri or Mumar. Those who forbid discuss a Yisrael.

Shulchan Aruch (YD 159:2): One may lend to a Mumar with Ribis, but one may not borrow from him with Ribis.

Taz (3 and Shach 3): The Tur says that one who pays Ribis to him transgresses Lifnei Iver, i.e. even Lifnei Iver, and also the Lav of paying Ribis to a Yisrael.

Beis Yosef (DH v'Havi): The Rosh and Tur discuss one who denied Torah. The same applies to one who transgresses to anger Hash-m.

Rema: Some are stringent even about lending to him. It is good to be stringent if one can evade him.

Gra (4): Even though he sinned, he is a Yisrael. The first opinion holds that even though he is a Yisrael, he is not your brother.

Shulchan Aruch (4): Kara'im (people who rejected oral Torah) are not considered Mumarim. One may not lend to them with Ribis, and all the more so one may not borrow from them with Ribis.

Beis Yosef (DH ul'Inyan): The Rambam says that Kara'im are like a captured baby, so one may not charge them Ribis. The Nimukei Yosef says that perhaps they are like Mumarim, for they are among Yisrael and choose to act like Nochrim. We do not abandon the Rambam's explicit opinion due to the Nimukei Yosef's Safek.

Shach (3): The Rambam says the opposite! The Poskim learned that one may lend to Mumarim with Ribis, and the same applies to Kara'im. The Maharshal and Drishah say so. R. Betzalel (Teshuvah 3) says that the Kara'im in the days of the Rambam had some good traits, unlike those today.

Note: Mishneh l'Melech (Hilchos Malveh 5:2) and Chidushei Hagahos on the Tur (1) explain Perush ha'Mishnayos like the Shach, and says that the text of the Beis Yosef should cite the Rambam in Hilchos Mamrim; Perush ha'Mishnayos discusses a Yisrael who became a Kara'ite. (Our text of Perush ha'Mishnayos distinguishes between the initiaters and those born to Kara'im, similar to Hilchos Mamrim - PF.)

Nekudas ha'Kesef: In Hilchos Mamrim, the Rambam says like the Beis Yosef says. It is not clear how to rule.

Rema: A baby captured by Nochrim who is totally ignorant of Torah law is like a Kara'ite. One may not lend to him with Ribis. Therefore, if a Mumeres married a Nochri and has a son from him, the son is a (Yisrael) like his mother, and he is a Mumar. One may not lend to him with Ribis, for he is like a captured baby raised by Nochrim.

Rema (160:1): One transgresses only Lifnei Iver for Ribis mid'Rabanan.

Pischei Teshuvah (1): The Mishneh l'Melech (Hilchos Malveh 4:2) says that Pnei Moshe says that if a borrower sees that the lender would lend the money to another Yisrael, Lifnei Iver does not apply. We say in Avodah Zarah that there is no Lifnei Iver when the wine is on the same side of the river. The Mishneh l'Melech disagrees. If no one gave to the Nazir, no Isur of Lifnei Iver would be done. Here, if the borrower does not borrow, another borrower would transgress Lifnei Iver, so there is no exemption from Lifnei Iver. (Note: seemingly, this is because we do not assume that the other borrower would transgress. However, Pischei Teshuvah considers that to be another reason! - PF) Also, perhaps the other borrower would retract and not borrow. The Rema (151:1) brought the opinion of Tosfos, the Ran and the Rosh that Lifnei Iver applies to mid'Rabanan even when the sinner could transgress by himself, for one must stop him. Even if this case is similar, it is forbidden l'Chatchilah, even for an Isur mid'Rabanan. However, regarding a Mumar, if it is like Nazir, it is permitted l'Chatchilah, like the Shach said. If the other Yisrael wants to borrow with Ribis for Piku'ach Nefesh or for a Mitzvah, which is permitted, Lifnei Iver would not apply, so also the first borrower is fully permitted. We are not concerned lest the other borrower retract, for it is permitted for him. However, we should forbid lest the first borrower prevent the latter from fulfilling Piku'ach Nefesh or his Mitzvah.