She fasted to expiate for breaking an oath, and she did not know that she should have fed poor people
Question

                I have fasted to expiate for my broken oath, then I heard that the last option is fasting and that I should have paid money, should I give sadaqah for each day I fasted?.
Praise be to Allah.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”
[al-Maa'idah 5:89]
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? 
He replied: 
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.