Charles Ellicott Commentary Numbers 30:5

"But if her father disallow her in the day that he heareth, none of her vows, or of her bonds wherewith she hath bound her soul, shall stand: and Jehovah will forgive her, because her father disallowed her." — Numbers 30:5 (ASV)

The Lord shall forgive her—that is, she would not incur the guilt or punishment which would otherwise have been incurred by neglecting to fulfil the vow which she had made.