The Gemora states that Rav also ruled that the child is fit. There was once a man who came to Rav and asked him: "What is the law regarding a Canaanite slave or an idolater who cohabits with a Jewess?" Rav said: "The child is fit." The man said: "If so, give me your daughter to marry (the man was from such a union)." Rav replied: "I will not give her to you." Shimi bar Chiya said to Rav: "People say, 'A camel in Medea dances in a kav (a camel can dance with its four legs in a small jar - an saying which means that people invent far-fetched stories from far away places that cannot be substantiated).' We have the kav and the camel, and this is Medea, but it is not dancing (if you ruled that it is permitted, give your daughter to him in marriage)." Rav answered: "Even if he were as great as Yehoshua son of Nun, I would not give my daughter to him." Shimi told Rav: "If he were as great as Yehoshua son of Nun, even if you will not give him your daughter, others would certainly give him theirs; however, regarding this man, if you do not give him your daughter, nobody will." Rav refused to change his mind and the man did not leave Rav. Rav gazed upon the man, and he died.
The Gemora states: Rav Masneh also ruled that the child is fit. Rav Yehudah also permitted the child. A man from such a union came to Rav Yehudah and Rav Yehudah said to him: "Go to a place where they will not recognize you, and you can marry a Jewess, or stay here and marry someone of your same type. Rava told the man the same advice.
(a) Could Rav have said, "Even if he is as great as Moshe Rabbeinu"?
(b) Why didn't Rav want to give his daughter to him if he himself ruled that the man was qualified?
(c) Why did Rav have him killed?
(d) How could they give advice to go to another city where they will not be recognized? Shouldn't this be gneivas da'as?
(e) Does lifnei iver apply by a chumrah?
(f) Can you sell or give someone something that is kosher, but not according to all opinions?
(a) The TAZ and BI'UR HA'GRA (YD 242:36) explain that Rav didn't mention Moshe Rabeinu, since that would be considered disgracing Moshe (since the Torah tells us that there cannot be any other Navi like Moshe).
(b) Qualified he was - i.e. permitted. But his Yichus was not the best, and we are told (Kidushin 70b) that the Shechinah only rests upon the families in Yisrael that are Meyuchasos, i.e. that trace their ancestry to Avraham Yitzchak and Yaakov.
(c) Rav did not intend to kill him. Hakadosh Baruch Hu stopped him in this manner from pestering Rav, in order to prevent a Chilul Hash-m.
(d) We hope to have an answer to this question shortly.
(e) Most Poskim (unlike Sha'ar ha'Melech, Hilchos Ishus 9:17) rule that Lifnei Iver does not apply to a Chumra. After all, if a person really holds something is permitted, how can he transgress Lifnei Iver when he gives it to someone else? The fact that he himself is stringent does not prohibit people from following the basic Halachah (see end of Teshuvos Ksav Sofer OC #66).
(f) This is a more complex question, as it depends on the reason that the item is not considered Kosher according to these opinions. In today's world, there are many reasons why people will not buy food bearing certain Hechsherim. People might not eat from a certain Hechsher because they do not approve of their general policies (i.e. amount of time Mashgiach is present as opposed to what is Kosher and what is Treif), not because they hold the food is unacceptable. This might mean they do not buy the product for their homes, but would eat it if it is served to them. Additionally, many people will eat at a Kiddush where people of their ilk are eating, and rely on the probability that the Hechsherim being served to them are reliable. In both of these cases, as long as the Hechsherim used are normally accepted among the people attending (at least b'Di'eved), it does not seem to be a problem to serve them such food (especially at a Kiddush where many different people prepare the food, as people who eat at such events know that they are relying on the kashrus standards of that particular populace). However, if a person holds eating a certain food is a Torah prohibition according to the letter of the law, it is prohibited to knowingly serve it to him (a Rabbinic prohibition is discussed by the Poskim). If the person serving the food holds it is absolutely forbidden and the person being served the food holds it is permitted based on the ruling of a Rav, it is generally permitted to serve the food (see Ksav Sofer YD 77 and Sefer Lifnei pg. 62). This is especially true if the issue is a Rabbinic prohibition (see Igros Moshe EH 4:61 and Minchas Shlomo #44).
The rules of when something is a transgression of Lifnei Iver or Mesayei'a are complex, and can easily change according to the situation. In all cases one should consult with a competent Rav.
Hatzlachah Rabah,
Yaakov Montrose and Mordecai Kornfeld