The ruling on one who swears that something will not happen, but it does happen

This is a letter we received from Muhammad al-Mutayri from al-Zulfi, the northern district. The brother Muhammad says in his letter: My mother swore by the Great Allah that her daughter would not take a man whom I wanted to marry to my sister. After a while, one of my mother’s relatives, a relative of my mother, asked for my sister and married her to the man about whom my mother had sworn that he would not take her daughter. They contracted the marriage for him, and after that it became clear that the one we married her to was a suckling infant to my sister, so they released her. The question is: Does she have to pay an expiation or anything else? She is completely embarrassed before her relatives because of the marriage of the aforementioned person. Please inform us, may Allah reward you.
Answer: She has to pay the expiation for the oath; because what she swore about happened. She swore that this man would not marry, that her daughter would not be married to this man, and then she allowed it and agreed to it, or she married her off without being pleased with it. In any case, as long as she married her to him, she has to pay the expiation for the oath. It is feeding ten poor people or clothing them. Each poor person is given half a sa' of dates, approximately one and a half kilograms of dates, or of rice, or of any type of the country’s staple food, or you clothe them with a garment, such as a shirt or an izar and a rida. This is the expiation. Or freeing a believing slave or a believing female slave. Yes.
The presenter: May Allah reward you.