The ruling on one who is unable to fulfill a vow due to old age or the like

Question:
I have a relative who married a man she does not love, and she was coerced into it. She then vowed that if he divorced her, she would fast two consecutive months. He then divorced her. What should she do? And what if she is unable to fast?

Answer:
This fasting remains a debt in her liability until she is able to perform it. The Prophet (peace and blessings be upon him) said: . So when she is able, she fasts. Otherwise, it remains a debt in her liability. If she becomes unable—for example, if she grows old in age, or if she is unable at that time—she must feed one poor person for every day, just as is the ruling for one who is unable to fast the month of Ramadan or days of it due to inability from old age, or a chronic illness that is not expected to be cured.