QUESTIONS: The Beraisa quotes Raban Gamliel who rules that even Nedarim which do not actually take effect, such as a Neder a woman makes not to serve her husband in one of the ways of endearment, still requires annulment. Raban Gamliel derives this Halachah from the verse, "Lo Yachel Devaro," which implies that even if a statement does not constitute a genuine Neder and is no more than a "spoken word," it requires Hafarah lest people become accustomed to taking Nedarim lightly (RAN, DH Mishum she'Ne'emar). He adds that the verse is merely an Asmachta, and the requirement of Hafarah in this case is only mid'Rabanan.
The Gemara cites another Beraisa which states, "[Where] certain practices are permitted but people regard them as prohibited, you have no permission to act in a way which permits those acts [in that place] in order to reject the conduct of those who prohibit them." (The Gemara earlier (15a) cites the same Halachah with a slightly different wording: "[Where] certain practices are permitted but people regard them as prohibited, you have no permission to permit those practices in front of them.")
The Gemara explains that this Halachah follows the view of Raban Gamliel that even a statement (or practice, in this case) which is not a genuine Neder requires Heter or Hafarah.
Based on the Gemara here, the SHULCHAN ARUCH (YD 214:1) rules that if a person is accustomed to fasting during the days of Selichos or between Rosh Hashanah and Yom Kippur and one year he decides not to fast because he is weak, he must seek a Heter to permit himself to eat on those days. Even if he fasted only one time in the past but intended that it be his accepted practice forever, he needs a Heter unless he specified that he was not accepting the practice upon himself as a perpetual obligation.
(a) Is the requirement of Hatarah for one who conducted himself stringently with a certain practice a Halachah mid'Oraisa or mid'Rabanan?
(b) Which types of practices are included in the requirement of Hatarah?
(a) The Gemara's comparison of Minhagei Isur with Nedarim which do not take effect implies that just as those Nedarim are Asur mid'Rabanan, so, too, Minhagei Isur are Asur only mid'Rabanan and require Hatarah only mid'Rabanan. (It is interesting to note, however, that although the rule of "practices that are permitted..." is the Halachah, the ruling of Raban Gamliel is not the Halachah, an indication that they are not totally comparable.)
The BIRKEI YOSEF (YD 214) indeed rules that the requirement for Hatarah is only mid'Rabanan. He points out that mid'Oraisa a Neder must be expressed verbally (Shevuos 26b), and thus a Minhag of Isur is binding only mid'Rabanan. He writes that this is implied by the wording of the Shulchan Aruch who writes that "it is as if he made a Neder," but not that it is considered an actual Neder.
It is not clear why the Birkei Yosef does not cite the Gemara earlier in Nedarim (15a) which states explicitly that the Isur of "Lo Yachel Devaro" in this case is only mid'Rabanan.
(b) This Halachah applies to both personal customs (which an individual accepted upon himself, such as fasting on certain days) and public prohibitions or "Minhagei Isur" (stringencies which all of the members of a community accepted upon themselves, such as the Minhag not to work on Erev Pesach). In both types of cases, one needs a Heter Chacham to discontinue the Minhag.
Some authorities maintain that one needs a Heter only when he was aware at the outset that the practice was merely a stringency and not the letter of the law. If one observed a prohibition because he mistakenly thought that it was the Halachah according to the letter of the law, he does not need a Heter Chacham (SHULCHAN ARUCH YD 214:1, first opinion).
Others, however, rule to the contrary -- that only if one mistakenly accepted the practice may he have a Chacham be Matir it. When he accepted the stringent practice knowing that it was permitted by Halachah, he may not have a Chacham be Matir it (TESHUVOS HA'RASHBA 1:98; RAN in Pesachim, Perek 4, second opinion, based on the Yerushalmi).
The VILNA GA'ON (to Shulchan Aruch, ibid.) cites the RAN in Pesachim who writes that the Gemara here provides support for the first opinion. The Gemara teaches that "Lo Yachel Devaro" is the source for the requirement of Heter for a stringent practice which a person accepted upon himself. This source implies that such a practice is comparable to a Neder. When a person makes a Neder, he knows that an object is permitted and he consciously attempts to prohibit it.
The Gemara in Pesachim (51a) cites the Beraisa which the Gemara here (and 15a) cites, "[Where] certain practices are permitted but people regard them as prohibited, you have no permission to permit those practices in front of them." Rav Chisda qualifies this rule and says that it applies only to permitting the practice in front of "Kuta'i" (unlearned people who follow practices out of ignorance), because when such people they see a person act leniently, they will treat all Isurim lightly; they will think that acts which are prohibited are really permitted. Why does the Shulchan Aruch record this ruling as though it applies to all people?
The VILNA GA'ON (to Shulchan Aruch, ibid.) answers that this single statement teaches two different Halachos (as the BEIS YOSEF mentions). The first Halachah is that one may not permit the practice without a Hatarah; this Halachah applies to everyone. The second Halachah is that one should not conduct himself leniently in front of others who conduct themselves stringently, lest they come to deride their own practice. The latter Halachah is the subject of the Gemara in Pesachim which applies only in front of "Kuta'im."