Allah Ta'ala has granted permission to people who are aged or terminally ill and have no hope of fasting again in their life, to compensate for each fast by feeding a poor and needy person (two complete meals for a day). This compensation is referred to as 'fidya.’
The Jurists have equated this amount to that of Sadaqatul Fitr (amount given before Eid to the poor). The current fitra amount for Ramadan 1436/2015 in KwaZulu-Natal, South Africa is (Sallallahu Alaihi Wasallam) 18.00 or more for each fast. Fidya may be given at the beginning of Ramadan or at the end. Fidya cannot be discharged before Ramadan. (Shaami).
This rule applies only to those who cannot fast because they are chronically ill and very old – with no hope whatsoever of fasting again.
However, those who were excused from fasting because of a valid temporary excuse must fulfil the missed fast(s) after Ramadan as soon as they have regained health and are able to. Fidya will not suffice in this situation.
It is also advisable to make a bequest that if missed fasts are not fulfilled in one's lifetime, then fidya should be given on one's behalf.