Sforno on Numbers 30:6/ספורנו על במדבר ל׳:ו׳

1וה' יסלח לה, for having made a vow which it was not in her power to fulfill

2כי הניא אביה אותה, and at the time she made the vow she had not known that her father would object to it. It had been her intention to honour her vow.