What is the ruling on a Christian woman, married to a Muslim man, vowing to fast Ramadan for five years if she gives birth to a boy, then she converts to Islam and is unable to fulfill the vow, and what are the consequences for her? And what is the ruling on vowing an act that is originally an obligation?

The vow of a disbeliever is valid and must be fulfilled if he embraces Islam, due to the Prophet's (peace and blessings be upon him) saying to Umar ibn al-Khattab (may Allah be pleased with him) when he made a vow in the pre-Islamic era: "Fulfill your vow." As for a person vowing to do something that is already obligatory upon him, such as fasting or praying, this vow is not valid and is not binding according to the jurists, and there is no expiation for it, because it is an undertaking of what is already obligatory by the original divine law. Therefore, nothing is required of the woman mentioned.