وإن لم يعين الشرع له وقتاً فإنه لا يتعين الوقت له بتعيين العبد.
When the law has not assigned a fixed time for an act, an individual's selection does not independently give it that status.
For a make-up Ramadan fast, the original duty does not identify one particular date of the year as the sole allowable day. On a valid day for make-up fasting, intention must identify the duty being discharged.
Example: A person plans to make up Ramadan on Wednesday but fasts on Thursday instead. The change of personal schedule does not extinguish the underlying make-up obligation.
The contrast between an original Ramadan day and a make-up fast on an unassigned day is essential.