Is it permissible to fulfill a vow from the orphans' money?

It is not permissible to fulfill a vow (nadhr) using the property of orphans, because Allah says: "And do not approach the orphan's property except in a way that is best." There is no benefit for the orphan in fulfilling a vow from their property; rather, it is considered theft. Whoever is in possession of an orphan's property must manage it in a way that is most beneficial for them. It is not permissible to dispose of it in any other way, even if they are the children of the one who is giving out charity.