If someone vowed to fast without specifying a time, or swore an oath and did not fulfill it, and then Ramadan began, is he required to feed a poor person for each day along with fasting, or is feeding sufficient and fasting is waived, or can he fast whenever he wishes and feeding is not required?

If one vows to fast for a day or a month, but does not specify it, or specifies it but does not define the time of the fast, then it is obligatory for him to fulfill his vow. It becomes a debt upon him if a period passes during which he could have fasted what he vowed but did not, or if the specified time for what he vowed passes and he did not fast.

Ibn Hajar Al-Haytami said: "If one vowed to fast on a Thursday but did not specify it, then any Thursday would suffice. If a Thursday passes, meaning he could have fasted on it, then, based on what has been mentioned regarding fasting, it becomes a debt upon him such that even if he dies, expiation is given on his behalf. There is no expiation for him for this fast, even if several Ramadans pass over him. He does not feed the poor unless he becomes unable to fast after it has become a debt upon him."

If the fast has not become a debt upon him, meaning a period during which he could have fasted did not pass and he did not fast, and he becomes unable to fast, then he only owes an expiation for breaking an oath.