She fasted to expiate for breaking an oath, and she did not know that she should have fed poor people
The expiation for breaking an oath consists of options in the order in which Allaah mentioned them in Soorat al-Maa’idah, where He says (interpretation of the meaning):
“Allaah will not punish you for what is unintentional in your oaths, but He will punish you for your deliberate oaths; for its expiation (a deliberate oath) feed ten Masaakeen (poor persons), on a scale of the average of that with which you feed your own families, or clothe them or manumit a slave. But whosoever cannot afford (that), then he should fast for three days. That is the expiation for the oaths when you have sworn. And protect your oaths (i.e. do not swear much). Thus Allaah makes clear to you His Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.) that you may be grateful”
So he should choose one of these three options and do it: feeding ten poor person with the average kind of food that he feeds his family, or clothing them, or freeing a slave. The one who does one of these things has fulfilled his duty and done what was required of him. If he is unable to do any of these three things, then he moves to the option of fasting, and he must fast three days.
It is not permissible to move to the option of fasting when one is able to feed or clothe poor persons or free a slave, because Allaah says (interpretation of the meaning): “But whosoever cannot afford (that), then he should fast for three days.”
Ibn al-Mundhir (may Allaah have mercy on him) said:
They (the scholars) are unanimously agreed that if the one who swears an oath can afford to feed or clothe ten poor persons or free a slave, then it is not acceptable for him to fast if he breaks his oath. End quote.
Al-Ijmaa’ (p. 157).
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked about a man who swore to do something, then he broke his oath, and he fasted even though he could afford to feed poor persons. What is the ruling? Is his fasting acceptable even though Allaah started with mention of feeding the poor and stated that fasting is to be done only if one cannot afford it? What if he was unaware of the ruling – does the ruling differ in that case?
If this person fasted as expiation for the oath when he could afford to feed ten poor persons or clothe them or free a slave, then his fasting is naafil, and he still has to offer the expiation. But his fasting is not wasted; it will count as naafil for him, and he should feed ten poor persons.
It is a widely-held view that expiation for breaking an oath (kafaarat yameen) is fasting. Hence when a person swears an oath to his brother and says “By Allaah, you must do this”, the other person says, “Do not make me fast three days.” But this is wrong. Feeding or clothing poor people or freeing a slave take precedence; but if he cannot afford to do that then he should fast for three consecutive days. End quote.
Al-Liqa’ al-Shahri (no 70, question no. 10).
In the answer to question no. 42804 we quoted a fatwa of the Standing Committee for Issuing Fatwas concerning this issue.
What you have to do is to feed ten poor persons for breaking your vow.
For more details on the expiation for breaking a vow, please see the answer to question no. 45676.
It should be noted that the expiation for breaking a vow cannot be paid in cash according to the majority of scholars. This has been discussed in the answer to question no. 89954. It must be given in the form of food or clothing.
And Allaah knows best.