Is it obligatory to fulfill a vow to refrain from taking money from one's husband after his death, if the husband was stingy and did not spend on his wife and children?

This vow is not binding, and it is not obligatory to fulfill it, because it is not something by which the Face of Allah is sought, and because it is a relinquishment of something that was not due to you, so it is like donating something one does not own.