All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.
Such a vow is not considered valid or binding, and you are not obliged to fulfill it, according to the preponderant opinion of scholars.
The Maaliki scholar Ad-Dusooqi said, about vowing, "A vow is considered binding and valid when the person vows to do an act that is recommended and the person intends to draw closer to Allaah Almighty by doing it …" [Sharh Mukhtasar Khaleel] The Prophet, sallallaahu ‘alayhi wa sallam, said: "There is no vow except in acts that are meant to draw one closer to Allaah, and no oath should be taken to sever the ties of kinship." [Abu Daawood and others; Al-Albani: Hasan]
Such a vow is not valid because you are relinquishing your due right in your husband's inheritance; it is a relinquishment of a right that is not yours yet. You are giving up your right before being entitled to it and before it is due. Scholars described this act as someone offering something that they do not own as a gift.
To conclude, you are not obliged to deliver on your vow because it is not considered valid and binding in the first place.
Allaah Knows best.