If a person owes two missed Ramadan fasts and also a separate expiatory fast, an unspecified “some fast” cannot simply be relabelled afterwards to discharge an obligation.
For liability-based fasts, the intention must identify whether the day is qada or kaffarah. That identifying intention must have been made in the preceding night.
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Kitab al-Sawm: Categories of Fasting and Rules of Intention
If a person owes two missed Ramadan fasts and also a separate expiatory fast, an unspecified “some fast” cannot simply be relabelled afterwards to discharge an obligation.
For liability-based fasts, the intention must identify whether the day is qada or kaffarah. That identifying intention must have been made in the preceding night.