What is the ruling of the Shari'ah regarding a mother's vow that her daughter donate half of her salary, and what must be done concerning the current obligations?

If your wife's mother vowed to donate half of what her daughter collects, this vow is invalid; because it is a vow concerning something she does not own. The Prophet, peace and blessings be upon him, said: "There is no fulfillment for a vow made in disobedience [to Allah], nor for something that a servant does not own." The mother is not required to offer expiation according to the majority of scholars. Some scholars said that it is forbidden to make a vow involving another person's property.