If he fulfils his vow, is it permissible for him to eat from it?
The basic principle concerning vows is that if what is vowed is something that is permitted in Islam, it should be given in the manner specified by the one who made the vow. If he did not specify anything then it is a kind of charity and may be given in any way in which charity is given, such as to the poor and needy.
With regard to eating from it, if the custom in the country where he lives is for the one who makes a vow of giving something edible to eat from it, it is permissible for him to do so, based on the local traditions and customs. Similarly, if he intended when he made the vow to eat from it, then both custom and intention apply to the part that he eats, and that is not part of that concerning which the vow was made. A fatwa was issued by the Standing Committee concerning that, which states the following:
The way in which a vow is to be fulfilled is to give in the manner intended by the one who made the vow, within the limits of sharee’ah. So if he intended the meat mentioned in his vow to be given to the poor, it is not permissible for him to eat from it, but if he intended in his vow to give it to his family or to a group of whom he is a member, then it is permissible for him to eat as one of them, because the Prophet (peace and blessings of Allaah be upon him) said: “Actions are but by intentions and each person will have but that which he intended.” The same applies if he stipulates that in his vow or if that is the custom in his country.
From Fataawa al-Lajnah al-Daa’imah li’l-Ifta, 23/390
And it says in Fataawa al-Lajnah al-Daa’imah (23/392): Whoever makes a vow to offer food, the basic principle is that the one who makes the vow should not eat from it unless he has stipulated or intended to eat from it, in which case it is permitted for him to eat as he stipulated or intended.
And Allaah knows best.