Hassidout sur Les Nombres 30:3

אִישׁ֩ כִּֽי־יִדֹּ֨ר נֶ֜דֶר לַֽיהוָ֗ה אֽוֹ־הִשָּׁ֤בַע שְׁבֻעָה֙ לֶאְסֹ֤ר אִסָּר֙ עַל־נַפְשׁ֔וֹ לֹ֥א יַחֵ֖ל דְּבָר֑וֹ כְּכָל־הַיֹּצֵ֥א מִפִּ֖יו יַעֲשֶֽׂה׃

Si un homme fait un vœu au Seigneur, ou s’impose, par un serment, quelque interdiction à lui-même, il ne peut violer sa parole: tout ce qu’a proféré sa bouche, il doit l’accomplir.

Kedushat Levi

Numbers 30,3. “when a man utters a vow, etc.;”
In ‎a book called Shaarey Orah, (a kabbalistic volume by ‎Rabbi Joseph ben Avraham Giktalia, earliest printing 17th cent.) ‎the word ‎נדר‎ is broken down into ‎נון דר‎, the letter ‎נ‎ referring to ‎Israel’s faith in G’d, Who in turn supervises our fate in all His ‎Mercy at all times and wherever we are, not abandoning us for ‎even a second. When this letter is found in the Torah written in ‎an inverted form, as in Numbers 10,35 or in Job 38,1, it points to ‎G’d’s supervision of the various parts of His universe. While the ‎letter ‎נ‎ in that verse in Job is not inverted but rather is a final ‎letter in the middle of a word, our sages in Nedarim 8 concluded ‎that a vow applies even to the promise to perform a ‎commandment when it is the result of the person making this ‎vow having experienced proof of G’d’s supervising His universe. ‎On such an occasion the person who has had this religious ‎experience decides to abstain even from matters which-as far as ‎the Torah is concerned- he is within his rights to enjoy.‎
‎[In order to understand this, it is necessary to ‎remember that the vows discussed in the Torah and the Talmud ‎involve use of the name of G’d, something severely punished ‎when used in vain, as per the third of the 10 Commandments. ‎Although the Talmud in Nedarim uses a verse from psalms ‎as the justification, our author feels that the remarks by the ‎author of the book ‎שערי אורה‎ reinforce what we have learned in ‎the Talmud. Ed.]‎

Kedushat Levi

‎“in order to impose a prohibition upon his ‎person.”
From the wording of the Torah it seems clear ‎that permission to utter vows or oaths is granted only if the ‎person doing so did so in order to strengthen his ability to obey ‎certain commandments that he was in danger of violating had he ‎not reinforced his determination by means of a vow or oath. An ‎example of such a vow is found in the Book of Ruth 3,13, where ‎Boaz reinforces his undertaking to be Ruth’s redeemer if a closer ‎relative refuses to marry her, by adding the words: ‎חי ה'‏‎, “by the ‎Life of Hashem”. He did so in order to bolster his resistance ‎to the evil urge that might advise him not to enter into such an ‎‎(apparently) inappropriate marriage.‎

Noam Elimelekh

"Or swears an oath" (Numbers 30:3) - meaning, shevuah (oath) comes from the expression "seven". A person needs to work through 7 levels, just as "six days you will do your labor, and on the seventh it is Shabbat". This is a hint that one needs to fix six corners, and after that one comes to the seventh level which is the holiness of Shabbat, and this is "swears an oath", meaning, the seven levels, each and every one of them made of of another seven.