What is the ruling if the vow-maker is observing a voluntary fast when the person arrives on the vowed day?

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If the voluntary faster is observing the vow day, Al-Qadi states that he continues the remainder of the fast, intending it for the vow, and it suffices without qada' or kaffara. This is the position of Abu Hanifa, based on the principle that part of the day can be considered voluntary and part obligatory, analogous to vowing to complete the remainder of a voluntary fast.