2 When a man vows a vow to Yahweh, or swears an oath to bind his soul with a bond, he shall not break his word; he shall do according to all that proceeds out of his mouth.

Matthew Henry's Commentary on Numbers 30:2

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.