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

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Fasting is not sufficient when one is able to feed poor persons for kafaarat al-yameen
What is kafaarat yameen in detail?

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Related Answers:
He offered kafaarat yameen in the form of cash; does he have to offer it again?
He vowed to offer an ‘aqeeqah for his son, but he was unable to fulfil his vow; what must he do?
The days do not have to be consecutive when fasting for kafaarat al-yameen
Fasting is not sufficient when one is able to feed poor persons for kafaarat al-yameen
What is kafaarat yameen in detail?

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