וַיְדַבֵּר מֹשֶׁה אֶל רָאשֵׁי הַמַּטּוֹת וכו' אִישׁ כִּי יִדֹּר נֶדֶר וכו' לֹא יַחֵל דְּבָרוֹ
And Moses spoke to the heads of the tribes. . . . If a man vows a vow to the Lord . . . he shall not break his word (Numbers 30:2-3)
See the Ramban who explains that this chapter was specifically transmitted to the heads of the tribes to indicate, as the Sages derive in the Talmud (Nedarim 78a-b), that they are authorized to nullify vows and oaths. The concept of invalidating vows has no basis in the Scripture (היתר נדרים פורחים באויר), and it hangs here only on a slender thread, so that the people should not take vows and oaths lightly. And he also wrote that perhaps it was necessary to keep the concept of invalidating vows hidden from them.
To enlarge on the words of the Ramban, our master said that the Torah did not suspect that the people would take lightly the vows that they make to the Eternal. On the contrary, the Torah commanded us not to desecrate our words to the Eternal to train us that all our words should be treated as holy - that we should take care to fulfill whatever we promise, even to a friend, that we not lie to another and that our "yes" and our "no" should be just. "For this is the whole duty of man" (כִּי-זֶה כָּל-הָאָדָם) (Ecclesiastes 12:13).
The Scripture therefore writes "he shall not break his word" (שלא יחלל דבריו), referring to our word to the Eternal so that we will learn to fulfill whatever we promise to our friend. The Sages relied on this verse for the concept of invalidating vows, because it is apparent from here that the reason that one may not violate his words to the Eternal is so that we may learn to fulfill whatever we promise to a friend. The Sages therefore inferred, "he may not break his word, but others may allow him to do so," because if it should become necessary to come to a sage to invalidate his vow, the sage will invalidate only the prohibitions that he imposed upon himself, but will not invalidate any promise made to another person.
However, since people might think that the sages are empowered to invalidate vows that have been made to another person - as, indeed, our enemies among the Gentiles say that we do to them and as Zedekiah, king of Judea, did when he commanded the Sanhedrin to annul the vow that he made to Nebuchadnezzar (see Nedarim 65a), the Scripture therefore hung the invalidation of vows on a slender thread and, as the Ramban wrote, explained it only through hints.
זֶה הַדָּבָר אֲשֶׁר צִוָּה ה' אִישׁ כִּי יִדֹּר נֶדֶר לַה'
This is the thing which the Lord has commanded (Numbers 30:2)
Rashi comments: "This is the word" (זֶה הַדָּבָר). It is a restrictive phrase intimating that the sage makes use of the expression "invalidating" (התרה), and the husband (concerning his wife's vows), the expression "annulling" (הפרה)
This is a forced deduction inasmuch as no "invalidation" of a vow is based on any Scriptural text. And were it not for the words of the Sages it would have appeared to our master to say that the verse: "This is the word that the Lord has commanded a man when he makes a vow unto the Lord" (זֶה הַדָּבָר אֲשֶׁר צִוָּה יְהוָה אִישׁ כִּי יִדֹּר נֶדֶר לַה') shows what verbal formulation a person should use in making a vow. And it appears to our master that our Sages relied on this to invent Aramaic terminology for vows, a language not understood by the ministering angels (מלאכי השרת), so that the Satan would not understand the vow and then denounce the one making the vow, for anyone who makes a vow is like one who builds a "high place" (כל הנודר כאילו בונה במה).
One can explain the relation between vowing and building a high place based on the saying of the Sages that one who is commanded and fulfills is greater than one who is not commanded but fulfills nevertheless (גדול המצווה ועושה ממי שאינו מצווה ועושה). To explain this saying, the Tosafot wrote that one who is commanded has a greater desire to transgress the commandment incumbent upon him than one who is not commanded, and his evil inclination is therefore more powerful, because (Proverbs 9:17): "stolen waters are sweet" (מַיִם-גְּנוּבִים יִמְתָּקוּ), and were it not for the assistance of the Holy One Blessed Be He, one could not withstand his evil inclination.
If so, by making himself one who is commanded, one who makes a vow incites the evil inclination within his soul. Why then should the Eternal help one such as this to conquer his evil inclination? For did he not bring everything upon himself? The Sages therefore properly compared one who makes a vow to one who builds a high place, as our master has explained at length elsewhere, and they adopted Aramaic terms for vows, so that the evil inclination would not understand and then seek to entice the one making a vow into breaking his word.
This is what is hinted at by the verse "אִישׁ כִּי יִדֹּר נֶדֶר לה'". It means "if one does make a vow, one should do so in a way that the vow will be known only to the Eternal." And the Scripture introduces the topic by explaining how this should be done by saying: "this is the word" as if to say that one should make a vow only with words such as these that are open only to the Eternal, which means that one should use the Aramaic terms that none but the Eternal - not even the angels - understands.
גִּדְרֹת צֹאן נִבְנֶה לְמִקְנֵנוּ פֹּה וְעָרִים לְטַפֵּנוּ
We will build sheepfolds here for our cattle, and cities for our little ones (Numbers 32:13)
Rashi comments: They paid more regard to their property than to their sons and daughters, because they mentioned their cattle before their children. Moses said to them: "Not so! Make the chief thing the chief thing and what is subordinate. First build cities for your little ones and afterwards folds for your flocks."
Our master interprets this verse in praise of the tribes and says that they did not pay more regard to their cattle than to their children, for did they not have dwelling places already built for them in the conquered land of Sihon and Og where their children and wives could be settled? However, these dwelling places were not fortified and not surrounded by walls. And if they would take up arms to go before the army, how could they leave their wives and children behind in open cities at the mercy of the surrounding inhabitants. They therefore suggested building sheepfolds for their cattle, which did not yet exist, and they would make the cities that were already there habitable by fortifying them with walls and latches. However, Moses, our teacher, reversed their proposal for a different reason, for perhaps they would suddenly be called upon to go before their brothers into battle before finishing building the walls around their cities. How could they then leave their children in such a vulnerable condition? He therefore commanded them to build the walls for the cities first, so that they would be ready to go out into battle with their brothers. That is why Moses said: "Do what you have promised. That is why I command you to build the cities first so that you will be ready at any moment to fulfill your promise."