If a validly begun voluntary fast is invalidated during the day, Quduri’s rule requires making up that one fast on another day.
This is qaḍāʾ rather than the special kaffārah attached to certain deliberate Ramadan violations. One should not transfer the Ramadan expiation automatically to a voluntary fast.
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Kitab al-Sawm: Interrupted Voluntary Worship, Puberty and Conversion
If a validly begun voluntary fast is invalidated during the day, Quduri’s rule requires making up that one fast on another day.
This is qaḍāʾ rather than the special kaffārah attached to certain deliberate Ramadan violations. One should not transfer the Ramadan expiation automatically to a voluntary fast.