Gemara

Question: Does Bal Yachel ever apply mid'Rabanan?!

Answer: (Beraisa): "Lo Yachel Devaro" - if something is permitted but people consider it to be forbidden, you may not be lenient about it to abolish the custom.

Pesachim 50a (Mishnah): In a place where the custom is to work on Erev Pesach until midday, one does so; in a place where people do not work, one must refrain.

If one goes from a place with one custom to a place with a different custom, he must be stringent like both places. He must follow the local custom to avoid strife.

Benei Beishan were stringent not to go from Tzur to Tzidon on Erev Shabbos (the market day, lest they not have time to prepare for Shabbos). The next generation was poorer, and felt unable to keep the stringency. They requested to permit it.

(R. Yochanan): Since your ancestors accepted this, you must follow it - "Shema Beni Musar Avicha v'Al Titosh Toras Imecha."

51a: Benei Chuzai used to separate Chalah from rice (bread).

Opinion #1 (Rav Yosef): A Zar (non-Kohen) should eat rice 'Chalah' in front of them (to show that it is not Chalah).

Question (Abaye - Beraisa): If people forbid certain things, you may not permit them to them.

Answer (Rav Yosef): Rav Chisda taught that this refers to Kusim (converts who observed only some Mitzvos).

Objection: One may not permit to Kusim, lest they come to be lenient about things that are truly forbidden. The same applies here!

Opinion #2 (Rav Ashi): If they eat mostly rice bread, a Zar should not eat rice Chalah in front of them, lest they (not separate anything and) forget the law of Chalah. If not, a Zar should eat rice Chalah in front of them, lest they separate Chalah from dough that is liable to exempt what is truly exempt, or vice-versa.

Question: Rav Chisda said that the Beraisa refers only to Kusim. This is not so!

(Beraisa): Two brothers may bathe together. In Kavul (a certain place), they do not. Once, two sons of R. Gamliel, Hillel and Yehudah, bathed together there. The whole region was in an uproar. Hillel (left the bath and) went to the outer room. He did not want to tell them that it is permitted.

Similar cases occurred in which Hillel and Yehudah once went out with light shoes on Shabbos in Biri, and R. Shimon ben Gamliel sat on a bench of Nochrim (on which merchandise is sold) on Shabbos in Ako.

When Rabah bar bar Chanah came to Bavel, he ate De'aisra (a certain Chelev of the stomach. Benei Eretz Yisrael eat it; Benei Bavel do not eat it.) Rav Avira Sava and Rabah bar Rav Huna came to see him; he covered it up.

Abaye (to Rav Avira and Rabah): He treated you like Kusim (he did not want you to see that it is permitted)!'

Chulin 110a: Rami bar Tamri came to Sura, and saw everyone discarding udders. He ate them. Rav Chisda questioned him. He said 'I am from Rav Yehudah's area (Pumbedisa), where we eat udder.'

Rav Chisda: One must observe the stringencies of his old and new residences!

Rami: I ate them outside the Techum (the area surrounding the city).

Rishonim

Rif and Rosh (2:3): If people treat something permitted like Isur, you may not be lenient and abolish the custom - "Lo Yachel Devaro." This is mid'Rabanan.

R. Yehudah ha'Kohen (cited in Hagahos Maimoniyos Shevu'os 12:3 and Rosh 4): We learn from here that if one used to refrain from meat and wine at a certain time, and he wants to retract, he needs Hatarah. Also the Yerushalmi connotes that it is a Neder. This is if he intended to always do so, like the case of others who are stringent about something. Even if he did so only once, he must permit it.

Rif and Rosh (Pesachim 17a and 4:3): Rav Chisda said that this refers to Kusim, for they will extrapolate and permit Isurim mid'Oraisa. It applies also to any place of ignoramuses, like the cases with R. Gamliel's sons. R. Yochanan forced Benei Beishan to keep their ancestors' stringency, due to "...v'Al Titosh Toras Imecha."

Question: How did Rebbi permit Beis She'an when he heard that R. Meir ate Yerek there without taking Ma'aser (Chulin 6b)? They were stringent!

Answer (R. Nisim, cited in Rosh): They thought that it was Eretz Yisrael. We may not permit only when they knew the Halachah and were stringent.

Rif and Rosh (ibid., citing the Yerushalmi): If one erred (and was stringent) and asked, we permit him, but not if he know that it is permitted and he was stringent.

Ba'al ha'Ma'or (Pesachim 16b): If one visits a place with intent to return to his hometown, he may be lenient like his hometown. If the new place knows that it is permitted, and they are stringent, he may not do so in front of them if there are no Chachamim there. If they erred, in any case one may permit it.

Rebuttal #1 (Hasagos ha'Ra'avad 1): The case in Bei Chuzai and Rav Ashi prove that even if ignoramuses err, one may not permit in front of them.

Rebuttal #2 (Milchamos Hash-m): One may not tell even Chachamim that it is permitted, for they must observe their customs! Rami bar Tamri needed to be stringent like the place he visited, even though he was from a place of Chachamim. The Yerushalmi forbids permitting a custom to be stringent, just like we do not permit a Shevu'ah to fulfill a Mitzvah.

Ba'al ha'Ma'or: Bnei Eretz Yisrael may not do Melachah on Yom Tov Sheni in Chutz la'Aretz in public, for this is a great custom that spread throughout all of Galus. One may not breach it.

Rebuttal (Hasagos ha'Ra'avad 2): We learn from Rami that one may not be lenient in the region of a bigger Chacham than himself. We often say 'it was Rav's region' to explain why one was stringent unlike his own opinion. If there is concern for strife, even in private one may not be lenient. If ignoramuses err, one may be lenient only in private. If ignoramuses ask, this shows that the custom is not so strong. Perhaps we permit them and explain everything so they will not err.

Ran (DH Yerushalmi): In many places the Bavli says that we did not permit people who asked because they were not Bnei Torah.

Rosh (ibid.): If one refrained from Heter for a fence or abstinence, this is like a Neder. A Chacham may permit him only like other Nedarim. He is Pose'ach with regret, i.e. that he acted so for the sake of a Neder. One must abide by customs of his new residence only if a Chacham there also did so.

Ran (Pesachim 17a DH Tanya): One must abide by customs of his new residence if they were a fence from Aveirah. If they erred, one may not be lenient in front of ignoramuses, but a Chacham may be lenient in his house.

Poskim

Shulchan Aruch (YD 214:1): If people are knowingly stringent about something permitted, it is like a Neder. One may not permit them. Therefore, if someone normally fasts before the Yomim Nora'im, or does not eat meat from Rosh Chodesh Av or the 17th of Tamuz, and he wants to retract because he is not healthy, three must permit him.

Question (Shach 2): The Rema agrees here, but regarding fasts of BaHaB and the 10 days of Teshuvah (OC 568:2), he does not require Hatarah if a Bris occurred, for there is no custom to fast in such a case. Likewise, there is no custom for one who not healthy to fast!

Answer #1 (Shach): There, he fasted with intent to eat if there will be a Bris. Here, he did not expect to be weak. We must say so, for the Rema there and the Shulchan Aruch here both come from Maharam! Likewise, no Heter is needed to eat meat and wine at a Seudas Mitzvah during the nine days.

Rejection (and Answer #2 - Dagul me'Revavah): This is difficult. Rather, there he eats at the Seudas Mitzvah, and returns to his custom. The same applies to if he was weak one day. Here we discuss one who seeks to cease fasting.

Shulchan Aruch (ibid.): If he began to do so with intent to do so forever, and he did so even once, he must permit it. We are Pose'ach with regret, i.e. that he did so to be a Neder. Therefore, if one wants to refrain from Heter for a fence or abstinence, he should say that he does not accept a vow, and that it is only for that time or as long as he wants, but not forever. If people are stringent about Heter because they think that it is forbidden, this is not like a Neder.

Beis Yosef (DH ul'Fi): If their ancestors knew that it is permitted and the children followed their stringency and thought that it is truly forbidden, like the case in Beishan, we permit only through Hataras Nedarim. If the custom began due to a mistake, we permit them automatically.

Beis Yosef (DH Hashta): If a custom began for the sake of a fence, one from another place may be lenient in front of them only if they are Bnei Torah. This is only distinction between Bnei Torah and ignoramuses.

Shulchan Aruch (ibid.): Some require Hataras Nedarim in this case, but if he knew that it is permitted and acted stringently, even Hatarah does not permit it. It is as if he accepted it on himself like an Isur Torah, which can never be permitted.

Source: Rashba (3:236, cited in Beis Yosef DH veha'Rashba).

Rema: The custom is like the first opinion.