Mon Nedarim 78

The Gemara says that a husband is only takes away a Neder if he uses the term "hafara." a Chachum only takes off a Neder with the term" Heter."

Ran explains: the term Heter means that it's permitted completely and there was never any Issur here. Therefore, since a Chachum removes the Neder retroactively, it's only appropriate to use the term "Heter". However, a husband only takes off the Neder from this time and on, but not retroactively. Therefore it's appropriate to use the term" Maifer" that implies that.