Biblical Commentary on the Old Testament, by Carl Friedrich Keil and Franz Delitzsh, [1857-78], at sacred-texts.com
The rules by which vows were to be legally regulated, so far as their objects and their discharge were concerned, has been already laid down in Lev; but the chapter before us contains instructions with reference to the force of vows and renunciations. These are so far in place in connection with the general rules of sacrifice, that vows related for the most part to the presentation of sacrifices; and even vows of renunciation partook of the character of worship. The instructions in question were addressed (Num 30:1) to "the heads of the tribes," because they entered into the sphere of civil rights, namely, into that of family life.
At the head there stands the general rule, "If any one vow a vow to Jehovah, or swear an oath, to bind his soul to abstinence, he shall not break his word; he shall do according to all that has gone out of his mouth:" i.e., he shall keep or fulfil the vow, and the promise of abstinence, in perfect accordance with his word. נדר is a positive vow, or promise to give or sanctify any part of one's property to the Lord. אסּר, from אסר, to bind or fetter, the negative vow, or vow of abstinence. על־נפשׁו אסּר אסר, to take an abstinence upon his soul. In what such abstinence consisted is not explained, because it was well understood from traditional customs; in all probability it consisted chiefly in fasting and other similar abstinence from lawful things. The Nazarite's vow, which is generally reckoned among the vows of abstinence, is called neder in Num 6:2., not issar, because it consisted not merely in abstinence from the fruit of the vine, but also in the positive act of permitting the hair to grow freely in honour of the Lord. The expression "swear an oath" (Num 30:2; cf. Num 30:13) shows that, as a rule, they bound themselves to abstinence by an oath. The inf. constr., השּׁבא, is used here, as in other places, for the inf. abs. (cf. Ges. 131, 4, note 2). יחל, from חלל, for יחל, as in Eze 39:7 (cf. Ges. 67, note 8), to desecrate (his word), i.e., to leave it unfulfilled or break it.
Num 30:3-15 contain the rules relating to positive and negative vows made by a woman, and four different examples are given. The first case (Num 30:3-5) is that of a woman in her youth, while still unmarried, and living in her father's house. If she made a vow of performance or abstinence, and her father heard of it and remained silent, it was to stand, i.e., to remain in force. But if her father held her back when he heard of it, i.e., forbade her fulfilling it, it was not to stand or remain in force, and Jehovah would forgive her because of her father's refusal. Obedience to a father stood higher than a self-imposed religious service. - The second case (Num 30:6-8) was that of a vow of performance or abstinence, made by a woman before her marriage, and brought along with her (עליה, "upon herself") into her marriage. In such a case the husband had to decide as to its validity, in the same way as the father before her marriage. In the day when he heard of it he could hold back his wife, i.e., dissolve her vow; but if he did not do this at once, he could not hinder its fulfilment afterwards. שׂפתיה מבטא, gossip of her lips, that which is uttered thoughtlessly or without reflection (cf. Lev 5:4). This expression implies that vows of abstinence were often made by unmarried women without thought or reflection. - The third case (Num 30:9) was that of a vow made by a widow or divorced woman. Such a vow had full force, because the woman was not dependent upon a husband. - The fourth case (Num 30:10-12) was that of a vow made by a wife in her married state. Such a vow was to remain in force if her husband remained silent when he heard of it, and did not restrain her. On the other hand, it was to have no force if her husband dissolved it at once. After this there follows the general statement (Num 30:13-16), that a husband could establish or dissolve every vow of performance or abstinence made by his wife. If, however, he remained silent "from day to day," he confirmed it by his silence; and if afterwards he should declare it void, he was to bear his wife's iniquity. עונה, the sin which the wife would have had to bear if she had broken the vow of her own accord. This consisted either in a sin-offering to expiate her sin (Lev 5:4.); or if this was omitted, in the punishment which God suspended over the sin (Lev 5:1).
Num 30:16, concluding formula.