The Gemara brings an example of a woman who takes a neder not to eat two loaves of bread. It turns out that refraining from eating one would not be a situation of innuy nefesh – suffering – while refraining from the other one would be innuy nefesh. Should her husband choose to nullify the vow – hafarat ha-neder – will it affect both loaves or only the single one that constitutes innuy nefesh? A husband is limited in his ability to do hafara only to situations where the neder affects the relationship between husband and wife or one where the woman who takes the vow would suffer innuy nefesh were she to fulfill it. Rav Yehuda quotes Shmuel as ruling that once one of the loaves is released from the vow, the other one is, as well. Rav Asi quotes Rabbi Yohanan as ruling that only the one that constitutes innuy nefesh is annulled, the other remains in force.
The question raised by the rishonim is how Shmuel can suggest that the husband can remove the neder that does not involve innuy nefesh – a power that he does not possess? Several approaches are offered in response to this question. One suggestion is that with regard to issues of innuy nefesh, once part of a neder is cancelled, the entire statement is rendered meaningless. Another approach is that, although our perception is that one loaf is desirable and missing out on it would be innuy nefesh while the other one is less desirable and is not considered innuy nefesh, there is some level of innuy nefesh in losing out on the ability to eat the second one as well.
The question on Rabbi Yohanan is presented in the other direction. Given the principle neder she-hutar miktzato, hutar kulo – a vow that is partially permitted becomes totally permitted – how can Rabbi Yohanan suggest that half of the woman’s neder remains in force? Rashi suggests that this principle applies only to a situation where the vow is hutar – is annulled by a rabbi. Only in that case is the neder cancelled retroactively and is perceived as never having taken effect. When a husband objects to a neder and is meifer, it only removes the part of the neder that is considered innuy nefesh, leaving the rest still intact.