The rabbis changed a Torah law
Numbers 30:3 contains the divine command, “A man who vows a vow to the Lord or swears an oath, to bind himself with a binding obligation, must not nullify his word; he must do all that goes out of his mouth.” This biblical law clearly forbids the cancelling of a vow; once someone utters an oath, he must carry it out, even if it results in death. This biblical rule forbidding nullification of vows was the view of all, or at least most ancient people.
Current Anglo-Saxon law allows the invalidation of vows based on a mistake and other reasons, so too does current rabbinical law because the rabbis changed the biblical mandate. The only cancellation process that is mentioned in the Bible is in Numbers 30, beginning with verse 4, which states that a father or husband can quash a daughter’s or wife’s vow. This exception existed because the Torah considered daughters and wives property of their fathers and husbands. What she has belongs to them, including her vows, and they control her vows. Even if she wants to fulfill her promise, her father or husband can stop her. Chapter 30 states that widows and divorcees, who are no longer under male domination, are like men in respect to vows; their vows cannot be rescinded.
I pointed out in the past that the judge Jephthah was unable to invalidate his vow to kill whatever comes to meet him after his battle and had to sacrifice his daughter. Three other tragic stories showing the inability to annul vows appear in the Bible. The patriarch Isaac’s blessing of Jacob could not be reversed even though it was given in error. Isaac wanted to bless his son Esau, but once he gave the blessing to Jacob he could not retract it. Joshua also was unable to renege on the promise he made to theGibeonites in Joshua 9:19. The tale of the tribe of Benjamin is a fourth instance: the tribes were unable to cancel their vow not to marry their daughters to the men of Benjamin.
The rabbinical handling of vows
Although the Torah is explicit in forbidding the nullification of vows, the rabbis stated that this was not the Bible’s intent, allowed the abolition of vows, and interpreted Numbers 30 as stressing the significance of vows which had not been nullified.
The fifth century Midrash Sifrei saw the verse speaking of the failure to carry out vows that had not been nullified. The Midrash stressed the sanctity of speech It pictured the person who violated his vow as a person who defiled a holy object.
The Babylonian Talmud, Shavuot 39a, saw the verse speaking about a false oath: The whole world trembled at the time when God said at Sinai: “Thou shalt not take the name of the Lord thy God in vain.” This is the kind of transgressor that the Torah says “The Lord will not hold him guiltless,” meaning, will not leave him unpunished. And moreover, for most other transgressions of the Torah, the sinner alone is punished, but in this case he and his family are punished; meaning that a false oath generally affects other family members, and others suffer the same consequences.
The twelfth century Midrash Numbers Rabbah even warns people not to swear truthfully. “If a man says to his friend: ‘I swear that I will go and eat a certain food at a certain place’….and even fulfills the oath, he will be destroyed. If this happens to one who swears truthfully, the consequences to one who swears falsely will be so much greater!”
The Midrash proves its point with an example. Alexander Yannai, a Hasmonean king around 126–76 B.C.E., lost two thousand of his towns because he swore a true oath. The Midrash states that people are only allowed to swear a true oath when three almost impossible conditions exist: (1) He must be as righteous as Abraham, Job, and Joseph. (2) He must devote all of his time to the study of Torah and the performance of its laws. (3) He must marry his daughter to a student who spends all of his time studying Torah and support him financially so that he does not have to neglect his studies.
The Bible clearly forbids the nullification of vows by men but allows husbands and fathers to nullify oaths of wives and daughters since these females are under the control of their husbands and fathers. The Torah was reflecting the view of the ancients that vows are sacred and cannot be annulled. This ancient notion changed in many cultures and the rabbis accepted the change. As a result, despite Scripture’s clear wording, the rabbis allowed the forgiveness of vows and interpreted the Torah mandate as an emphasis on performing what one promises to perform, unless the man cancels his vow.
 The laws of vows for men are also in Leviticus 5:4 and 5, Numbers 6, and Deuteronomy 23:19, 22-24. But this is the only site dealing with female vows.
 See Judges 11 and Genesis Rabbah 60:3.
 Genesis 27.
 See also the discussion in the Babylonian Talmud, Gittin 46a.
 Judges 21, see also Mishnah Gittin 4:7.