If one is noder hana'ah from something which sometimes is obligatory and sometimes is not, is the neder binding?

Question

Let's say I make a neder not to benefit from the five grains - wheat, barley, rye, oat(?), and spelt. These are the only grains which can be used for Matzah on Pesach (Pesachim 35b). Assuming that a neder cannot override a mitzvah d'Oraisa (that is, according to the Ramban cited there), is my neder binding?

• On the one hand, I can get away with that for most of the year without violating any absolute obligations, so maybe the one week a year on which it's mandatory is batel b'rov.
• On the other hand, maybe a neder needs to be able to never be against a mitzvah, and this would be against that rule.
• On the third hand, maybe it would be valid whenever it can be valid, and not whenever it isn't. Therefore, it would be valid during the year, but not on Pesach.

Answer 1

A Neder works to prevent a person fulfilling the actual Mitvas Asei like eating Matza itself by making it forbidden. How much more so it works to forbid the 5 grains which is forbidding the Mitzvah indirectly. Shulchan Aruch Yore Dea 215,1 based on Nedarim 15a:

נְדָרִים חָלִים עַל דְּבַר מִצְוָה. כֵּיצַד, אָמַר: קוּנָם סֻכָּה שֶׁאֲנִי יוֹשֵׁב, לוּלָב אֲנִי נוֹטֵל, אָסוּר לִישֵׁב בַּסֻכָּה וְלִטֹּל לוּלָב. הַגָּה: וְיֵשׁ אוֹמְרִים שֶׁמַּלְקִין אוֹתוֹ עַל שֶׁנָּדַר לְבַטֵּל הַמִּצְוָה, וּמַתִּירִין לוֹ נִדְרוֹ וּמְקַיֵּם הַמִּצְוָה (מָרְדְּכַי רֵישׁ שְׁבוּעוֹת שְׁתַּיִם בְּשֵׁם ר''ח)
Nedarim are binding to prevent one fulfilling a Mitzva. If he said "Konam (a vow) that any Sukkah should be prohibitted for me to dwell in, any Lulav forbidden for me to take," it is forbidden to fulfill those Mitzvos.
Rema: Some authorities allow one to annul that vow inorder to fulfill the Mitzva.

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Forgive me if i did not make myself clear in the post quoted, The Tosfos and Ramban hold one cannot make a Neder to make something forbidden which is already a Mitzva that forbids such an action e.g not eating bread on Yom Kippur because ein issur chal al Issur i.e you can't forbid something twice. but they agree with Nedarim 15a which states Explicitely a Neder can prohibit a positive Mitzva.