The Mishnah differentiates between a case where one makes a Neder
prohibiting all Kohanim from having Hana'ah from him, in which case the
Kohanim nevertheless may come and take Terumah from him without his consent,
and a case where one makes a Neder prohibiting only specific Kohanim from
having Hana'ah from him, in which case those Kohanim may *not* take his
Terumah. The Gemara points out the underlying element of contradiction
between these two cases: on one hand, the former case proves that "Tovas
Hana'ah" (the right of the owner to choose to which Kohen he will give the
Terumah) is not considered to have any monetary value ("Tovas Hana'ah Einah
Mamon"), and therefore the Kohanim may take the Terumah, for they are not
receiving any monetary benefit from the Madir. On the other hand, the latter
case, in which the specific Kohanim are prohibited from taking the Terumah,
seems to prove the opposite, that Tovas Hana'ah *does* have monetary value,
and that is why the Kohanim may not take the Terumah, for it is considered
as though they are receiving monetary benefit from the Madir.
Rebbi Hoshiyah answers that it is actually a Machlokes. The Tana of the
Reisha holds that Tovas Hana'ah does *not* have monetary value, and in both
cases the Kohanim may take the Terumah. The Tana of the Seifa holds that
Tovas Hana'ah *has* monetary value, and in both cases the Kohanim may *not*
take the Terumah. (The Acharonim differ with regard to the view of the Tana
of the Reisha (see TOSFOS YOM TOV Bava Kama 9:10); some maintain that since
the Tana holds that Tovas Hana'ah has no monetary value, the Madir may
actually give the Terumah directly to the Kohen, since the Kohen's benefit
is non-monetary, while others say that this Tana agrees that the Madir may
not give the Terumah directly to the Kohen, but only allows the Kohanim to
come and take it themselves.)
Rava answers that although the Mishnah holds that Tovas Hana'ah does have
monetary value, when a person's Neder includes all the Kohanim, that
person's Tovas Hana'ah is rendered worthless. Hence, the Kohanim may take it
from him.
The RAN explains that Rava means that since the Madir is left with no Kohen
to whom to give the Terumah, he has effectively made the Tovas Hana'ah into
Hefker. That is why anyone (who is a Kohen) may now come and take it.
Similarly, the ROSH and TOSFOS write that his Tovas Hana'ah has become
worthless because he is left with no one to whom he can give the Terumah,
and therefore the Kohanim can take it, because it is now worthless.
Most of the Rishonim write that Rava's answer, that the Tovas Hana'ah is
rendered worthless since the Madir prohibited all Kohanim from benefitting
from him, is true only according to the opinion that holds that Tovas
Hana'ah has monetary value. According to the view that Tovas Hana'ah has no
monetary value, Rava agrees that the Kohanim can take the Terumah regardless
of the scope of the Neder (that is, whether it applies to all Kohanim or to
just a few Kohanim).
The RAMBAM (Hilchos Nedarim 7:11) rules like Rava, that when the Neder was
made on individual Kohanim, they may not take the Terumah, even though he
also rules in several places (including Terumos 12:15) that Tovas Hana'ah
does *not* have monetary value.
The KESEF MISHNAH (in the name of Rabeinu Meir ha'Me'ili) writes that
although the Rambam holds that Tovas Hana'ah does not have monetary value,
still it is prohibited for the Kohanim to take the Terumah for even
non-monetary benefits such as Vitur (see 32b). This view seems to be
supported by the fact that Rava himself (Bava Metzia 11b) is of the opinion
that "Tovas Hana'ah Einah Mamon."
The KETZOS HA'CHOSHEN (275:1) reconciles this apparent discrepancy in Rava
(according to the Rishonim who explain that Rava is of the opinion that
Tovas Hana'ah has monetary value). Although Rava considers Tovas Hana'ah
significant enough to be prohibited to the Mudar Hana'ah, it is not
significant enough to be *transferable*. The Gemara in Bava Metzia is
discussing the transferal of Terumah through a Kinyan, which Rava holds does
not work for Tovas Hana'ah. Tovas Hana'ah is certainly not worthless; it is
just not significant enough to be transferred.