ONE IS MATFIS IN THE INITIAL ISUR [Nedarim: Hatfasah]
20b (Mishnah): If Reuven said 'I am a Nazir'; another (heard him and) said 'and I'; (and another said) 'and I', they are all Nezirim. If Reuven permitted his Nezirus, the Nezirus of all of them is permitted. If the last permitted his Nezirus, he is not a Nazir, but the rest of them are Nezirim.
21a - Question: Is everyone (after the second) Matfis (to forbid Heter like Isur) in Reuven (i.e. accepts to be a Nazir like him), or in the last one who spoke?
Answer (Beraisa): If the middle one permitted his Nezirus, only those before him are Nezirim.
This shows that each is Matfis in the last one.
Nedarim 11b (Rami bar Chama) Question: If a Chulin piece was next to a piece of Shelamim after Zerikah (throwing the blood on the Mizbe'ach), and he said "this (the Chulin) is like this", what is the law?
Is he Matfis in the initial Isur (before Zerikah), or in its (current) Heter?
Answer #1 (Rava - Mishnah): (If one said...) 'Nosar', or 'Pigul' (he forbids.)
These are after Zerikah, and it is still binding. This shows that one is Matfis in the initial Isur.
Rejection (Rav Huna brei d'Rav Noson): Perhaps the Mishnah discusses Nosar of an Olah (it is always forbidden). This is a bigger Chidush than meat of an Olah:
Not only Hatfasah in an Olah is forbidden, for surely he is Matfis in Davar ha'Nadur (something that became forbidden through a vow);
Rather, it forbids Nosar and Pigul of an Olah. One might have thought that he is Matfis in the Isur of Nosar or Pigul, i.e. Hatfasah in Davar ha'Asur (something forbidden, but not due to a vow), which does not forbid.
Answer #2 (Beraisa) Question: What is Isar mentioned in the Torah ("Lesor Isar Al Nafsho" - Bamidbar 30:3)? One said 'I will not eat meat or drink wine like the day my father died, or the day Gedalyah ben Achikam was killed...'
(Shmuel): The case is, he (once) vowed not to eat meat or drink wine on that day.
Suggestion: The case is, he said 'I am forbidden today like (e.g.) Sunday.' Even though there are many permitted such days, we assume that he is Matfis in the initial Isur (the day he vowed)!
Rejection: No, Shmuel means that he had already vowed not to eat meat... on every such day (so it is impossible to say that he was Matfis in Heter).
Nazir 22a (Beraisa): If Leah accepted to be a Nezirah and Rachel said 'and I', and Leah's husband annulled her vow, Leah is not a Nezirah, but Rachel is;
R. Shimon says, if Rachel said 'I am like you', when Leah's Nezirus is annulled, Rachel's is also.
(Mar Zutra, son of Rav Mari): We can resolve Rami's question from the first Tana. (Rachel is forbidden because one is Matfis in the original Kedushah.)
Version #1 - Objection: Our case is unlike Rami's case!
There, even though the meat is permitted after Zerikah, it may not be eaten outside the Mikdash! (It still has some Kedushah. Perhaps one is Matfis in the final Kedushah!)
Here, Rachel cannot intend to be Matfis in the final Kedushah. After Hafarah, Leah is not a Nezirah at all!
Version #2: Indeed, we can resolve Rami's question from the first Tana.
Rif (Shevuos 8b): Shmuel established the Beraisa to be when he had already vowed not to eat meat on every such day.
Ran (DH v'Omar): The case in Nazir (22b) is not exactly like Rami's question. Rami asked about Shelamim after Zerikah, which is permitted, like a woman after her Nezirus was annulled. Surely, Hatfasah in such a woman is void! Rather, Hatfasah in a Nezirah is like Hatfasah in Shelamim before Zerikah. Is one Matfis in its current status? Or, does he want the new item to always be like the Nitfas, even if the Nitfas becomes permitted? In Nedarim we asked whether the Chulin will be permitted after Zerikah of the Shelamim is done. Alternatively, we also asked about Hatfasah in a Shelamim after Zerikah. In any case, since the Stam Gemara equated them, we learn that Rami's question was resolved; one is Matfis in the initial Isur.
Ramban (on Rif Nedarim 4b): The question of whether one is Matfis in the initial or final status was not settled in Nedarim, but it was settled in Nazir, that one is Matfis in the final status.
Rebuttal (Ran, ibid.): Presumably, the Ramban refers to Nazir 21a. If a series of people were Matfis Nezirus, we concluded that each was Matfis in the last one before him. The Ramban held that the Gemaros that mention Rami's question contradict each other, so he did not rely on them. I disagree. It cannot be that the Gemara on 21a settles Rami's question leniently without mentioning it, and on 22b it explicitly settles his question stringency, and ignores the contradiction! Rather, Rami's question is unrelated to the case of Nezirim, in which there are distinct people, and we asked in which one is Matfis.
Ramban (ibid.): The Gemara in Shevuos cites Shmuel like the opinion that one is Matfis in the final status, and the Rif wrote so in his Halachos.
Rebuttal (Ran, ibid.): The Gemara used Shmuel to answer according to all opinions. We cannot infer that one is Matfis in the final status!
Rambam (Hilchos Nedarim 1:15): If Kodesh meat was in front of him, even of a Shelamim after Zerikah which is permitted to Zarim, and he said 'this produce is forbidden to me like this', it is forbidden, for he was Matfis in the initial status, which was forbidden.
Ran (ibid.): The Rambam holds that even though the question was not settled in Nedarim, Rava and Ravina, who are Basra, tried to prove that one is Matfis in the initial status. The Gemara rejected their proofs, but in any case they held this way, and we rely on them.
Rambam (3:5): If one vowed to fast on the day that his father died, and he did, and years later he said 'today is like the day that my father died', he may not eat, for he was Matfis it and forbade it like a day forbidden to him. The same applies to all similar cases.
Question (Lechem Mishneh): The Rosh and Ran explained that he did not specify 'like the day that my father died.' Rather, he said only 'like Sunday.' The Rambam (1:15) rules that one is Matfis in the initial Isur. He should teach this here!
Rosh (Shevuos 3:3): A Neder takes effect on an Isur mid'Oraisa and one transgresses "Lo Yachel." However, if he was Matfis something else in it, it is not forbidden, for he is Matfis in the initial Isur, so this is not Davar ha'Nadur.
Shulchan Aruch (YD 204:1): If one vowed to fast one day, or not to eat meat that day, and he said that another day should be like that day, it is forbidden.
Beis Yosef (DH u'Posak): The Rambam and Rosh rule that one is Matfis in the initial vow. However, R. Yerucham cites some who say that the question was settled in Nazir that one is Matfis in Heter. The first opinion is primary.