On yesterday’s daf we learned of the distinction between things that are considered davar she-yesh lo matirin (something that is forbidden today, but will become permitted at a later time) and things that are davar she-ein lo matirin (something that is forbidden forever). In the latter case, the concept of bittul (nullification when mixed with a larger volume of permitted food) applies, while in the former case, bittul does not apply: since it will become permitted simply with the passage of time, there is no pressing reason to employ the rules of nullification. Our Gemara questions whether something that is forbidden because of a vow can be considered a davar she-yesh lo matirin on the basis of the fact that a person can undo the vow by consulting with a rabbi and questioning the assumptions that he made while taking the neder – a method referred to by the Gemara as being sho’el on the neder.
While the Gemara considers the possibility that a neder should be considered a davar she-yesh lo matirin, it is clear that teruma is not considered a davar she-yesh lo matirin even though if a person believes that the produced was tithed in error he can go to a rabbi and be sho’el on the tithing. The Gemara distinguishes between the two by quoting Rabbi Natan who rules that someone who takes a neder is compared to a person who builds a forbidden altar and when someone fulfills his neder it is as though he brought a forbidden sacrifice on it. Thus, Rabbi Natan considers it a mitzva to annul one’s vows, while it is certainly not recommended for a person to annul his tithes.
Rashi explains that in the case of nedarim, even though the vow is still in force, since it is a mitzva to be sho’el we consider it as though it had already been annulled. The Rosh understands the reasoning as being that we work with the assumption that the neder will be annulled, so we consider it a davar she-yesh lo matirin.