The ruling on one who breaks an oath and is unable to feed or fast

Question:
A woman swore an oath regarding her daughter: that she would not leave the house. On the second day, she allowed her to leave, and she was unable to fast. Can her son fast on her behalf? It is known that she is unable to feed the poor.

Answer:
The expiation for an oath is to feed ten poor people, or clothe them, or free a slave. Whoever is unable to do this must fast for three days. If she is poor and cannot afford the expiation, and is unable to fast, the expiation remains pending; the expiation remains a debt in her record. Her son does not fast on her behalf; one does not fast on behalf of a living person. However, if he expiates on her behalf with food and she gives him permission, there is no harm. If she allows him to expiate on her behalf with food, or clothing, or freeing a slave, there is no harm. But fasting is not allowed. If he does not expiate on her behalf, the expiation remains. When she is able, she expiates by feeding, or clothing, or freeing a slave. And when she is able to fast, if she is unable to expiate by feeding or the like, she fasts: [al-Taghabun:16].