In the Insights to the Daf 12B, pshat in the two tzdadim of the Iboyah are explained that if B'ikrah K'Matfis then it's not a neder because it's a Davar H'Asur, however if B'Hetaira K'Matfis then it's Chal because he is being Matfis B'Davar Hanodur. To me it appears that the Rishonim seem to learn that if it's B'Ikrah then it's a good neder because it was originally Asur (and this Issur is Lichorah a Dovor Hanodur because it came about Al Yedai Neder) Ma Sheain Kain if it's B'Hetairah KaMatfis then he has no neder as he is being matfis in something that is Mutter. Please explain.

Dovid Maryles, Lakewood, NJ