The Law concerning Vows
301 Then Moses summoned the leaders of the tribes of Israel and told them, "This is what the Lord has commanded: 2 A man who makes a vow to the Lord or makes a pledge under oath must never break it. He must do exactly what he said he would do.
Matthew Henry's Commentary on Numbers 30:1-2
Commentary on Numbers 30:1-2
(Read Numbers 30:1-2)
No man can be bound by his own promise to do what he is already, by the Divine precept, forbidden to do. In other matters the command is, that he shall not break his words, through he may change his mind.