This week's Torah portion contains a striking parallel between how it begins and how it ends. The portion begins as follows:
2. Moses spoke to the heads of the tribes of the children of Israel, saying: This is the thing the Lord has commanded. 3. If a man makes a vow to the Lord or makes an oath to prohibit himself, he shall not violate his word; according to whatever proceeded from his mouth, he shall do.
ג. אִישׁ כִּי יִדֹּר נֶדֶר לַיהֹוָה אוֹ הִשָּׁבַע שְׁבֻעָה לֶאְסֹר אִסָּר עַל נַפְשׁוֹ לֹא יַחֵל דְּבָרוֹ כְּכָל הַיֹּצֵא מִפִּיו יַעֲשֶׂה:
24. So build yourselves cities for your children and enclosures for your sheep, and what has proceeded from your mouth you shall do."
כד. בְּנוּ לָכֶם עָרִים לְטַפְּכֶם וּגְדֵרֹת לְצֹנַאֲכֶם וְהַיֹּצֵא מִפִּיכֶם תַּעֲשׂוּ:
We shall build sheepfolds for our livestock here: They were more concerned about their possessions than about their sons and daughters, since they mentioned their livestock before [mentioning] their children. Moses said to them, “Not so! Treat the fundamental as a fundamental, and the matter of secondary importance as a matter of secondary importance. First ‘build cities for your children,’ and afterwards 'enclosures for your sheep’” (verse 24) - [Mid. Tanchuma Mattoth 7]
Another comment made by Rashi, perhaps not as famous, is on the phrase that links the beginning of the Parasha to its end:
and what has proceeded from your mouth you shall do: for the sake of the Most High [God], for you have undertaken to cross over for battle until [the completion of] conquest and the apportionment [of the Land]. Moses had asked of them only “and… will be conquered before the Lord, afterwards you may return,” (verse 22), but they undertook,“until… has taken possession” (verse 18). Thus, they added that they would remain seven years while it was divided, and indeed they did so (see Josh. 22).
Moshe is holding the Tribes accountable for an additional condition, which Moshe himself had not asked of them. Moshe asked that they stay until the Land be conquered, but they vowed to stay until the Land had been properly apportioned, which required that they stay an additional seven years.
The question is: how could Moshe hold them to this requirement if in fact they did not pledge to this in the form of a vow. All they said was, "We shall not return to our homes until each of the children of Israel has taken possession of his inheritance." One could even argue that they were still "negotiating" with Moshe.
From here we learn again, what is the main theme of the Book of Bamidbar: the tremendous power of words. (Midbar means desert, but has at its root Davar, word). One has to be so very careful about what one says, certainly involving the bad, but regarding the good as well. In Jewish law, any expression of willingness to perform a mitzvah or good deed brings upon an obligation.
Moshe is therefore able to take something that seemed abstract and perhaps even out-of-place in the outset of the parashah, and drive it home in the most practical of ways: a few added words led to a commitment to stay seven more years away from their families and livestock, and as Rashi concludes, "indeed they did so."