The ruling on one who vowed to fast for a full year

Question:
The brother has raised this question more than once and insists on presenting it. My wife’s grandmother was at Al-Shamisi Hospital, and she wanted to take a taxi with her sister. When they stopped the taxi driver, it turned out he was a relative of hers. She and her sister got in with him and gave him the fare, but he refused to take it from her. She then vowed that she would fast for a full year if he would take the fare from her, but he refused. What is the ruling on this oath she made upon herself to fast for a full year?

Answer:
She owes the expiation for an oath. This is a disliked oath, not in accordance with the Shari'ah, and she only owes the expiation for an oath. This is because her intention was not to fast; her intention was to compel him. So she owes the expiation for an oath: feeding ten poor people, or clothing them, and that is sufficient.