Matthew Henry Commentary Numbers 30:1-2

"And Moses spake unto the heads of the tribes of the children of Israel, saying, This is the thing which Jehovah hath commanded. When a man voweth a vow unto Jehovah, or sweareth an oath to bind his soul with a bond, he shall not break his word; he shall do according to all that proceedeth out of his mouth." — Numbers 30:1-2 (ASV)

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 should not break his word, though he may change his mind.