Fidya for Missed Fasts

Q: saum-4O: when can the Fidya be paid for missed fasts?

Answer: When a person is so old that he had no strength to fast and there is no hope of him ever recovering enough to fast, he may pay the Fidya. The Fidya amounts to approximately 1.75kg. of wheat flour for every fast missed. The equivalent cash may also be given, or even the equivalent value of clothing or religious books. If the person does recover the strength to fast before death, it will be necessary for him to observe the fasts he has missed.

Fatawa Rahimiyyah vol.3