Is it permissible to change the value of a vow?

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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

When one makes a vow, he is required to fulfill it according to the way that he specified it. It is impermissible to fulfill it in a way other than that which he has originally intended unless there is an outweighing benefit. For example, if one makes a vow to give food to the needy and he finds that giving them money which equals the food's value is more useful to them, then it is permissible to do so. However, if paying the value is not beneficial to the needy, then it is impermissible to pay the value, and the person is required to fulfill the vow exactly as he specified. 

Allaah Knows best.