Nedarim 75a - Source for not doing Hakama in advance

The Ran seems to assume that there isn't any source for the mishnas assumption that hakama cannot be done in advance, therefore the default position is that it cannot be done in advance. Regarding hafara the mishna quotes a machlokes, r' elazar holds that it can be done in advance based on a kal v'chomer, but the Rabonon hold that it cannot be done in advance either because we make make a hekesh between hakama and hafara, since hakama cannot be done in advance, so too hafara cannot be done in advance.

R' Akiva Eiger asks, if there would be a source for hakama no being able to be done in advance, the hekesh would enable us to learn hafara from hakama, but since there is no source to say that hakama cannot be done in advance, rather there is a kal v'chomer that hafara can be done in advance, the hekesh should teach us to learn hakama from hafara ans say that they both can be done in advance? In truth, the gemara quotes braisas where the Rabonon clearly reject the premise of R' elazar's kal v'chomer and are left without a source for either hafara or hakama, but if so the question may still be asked perhaps both hafara and hakama can be done in advance rather than saying they both cannot be done in advance?

The Rosh was bothered by this question and therefore agrees with R' Eliezer Mimitz that he cites. The source of hakama not being effective when done in advance is that since he will probably not be happy with all her nedarim, it is a hakama b'taos (mistake) and not binding. Based on this we have a clear source that hakama cannot be done in advance and learn hafara from hakama based on the hekesh, that hafara also cannot be done in advance. Based on the approach of the Rosh, the Rashash points out that if one would specify certain nedarim by saying "if she makes a neder later about.... then i am mikayem the neder from today", the concern of ta'os would not apply, and the kiyum would be binding even before the neder is made. However, the Rashash quotes a source from the sifri that learns from a pasuk that one cannot do hakama in advance of the neder.

Perhaps the Ran would hold that the concept of hakama is impossible to work before the neder is chal, since by definition it simply supports a preexisting neder, so if there isn't any neder there isn't any hakama. However, by hafara it is possible that it would work in advance either because it simply would prevent the neder from being chal or because the neder is born with a blemish (like the ran explains when an animal is born with a mum) so that the neder is immediately chal and then removes, as the gemara discusses. Therefore, it is only possible for hafara to work in advance, but impossible for hakama to work in advance. Once we have a hekesh saying the two must be the same, we are forced to say that neither one can be done in advance.