What is the ruling on a vow that obliges the questioner not to buy gold for his wife unless he buys an equivalent amount for his mother and sister, in a case where he has bought for his mother and sister and wishes to buy a gift for his wife as compensation for her?

This vow falls under the category of permissible vows, and according to the majority of scholars, nothing becomes binding because of it. However, according to the Hanbalis, one has the choice between fulfilling it or offering an expiation for an oath. There is no objection to giving a gift to the wife; in fact, it is among the causes of love and strengthens the marital bond, based on the saying of the Prophet, peace and blessings be upon him: "Exchange gifts, and you will love one another."