“[Fasting is for] a limited number of days. So whoever among you is ill or on a journey, then [he shall fast] a number of other days. And upon those who are unable to fast is a ransom: feeding a needy person. And whoever volunteers more good, it is better for him. Yet that you fast is better for you, if only you knew.”
— Qur’an, Surah Al-Baqarah 2:184
The above noble verse expresses the answer to the question. Accordingly, fasting during the month of Ramadan is obligatory upon a sane and legally accountable believer who enters the month. However, if one enters Ramadan while having an excuse that prevents fasting—such as illness or travel—then that person will make up those fasts later, equal to the number of days missed. If, however, one subsequently becomes entirely incapable of fasting in any way, only then must fidyah be given for each day that could not be observed.
Accordingly, a person who has the ability to fast, whether by performing it within its proper time or by making it up later, cannot free himself from the obligation of fasting by paying fidyah. Those for whom paying fidyah is permissible are individuals who are unable to fast both now and in the future. These are the very elderly and those suffering from illnesses for which there is no prospect of recovery. As for circumstances such as pregnancy, breastfeeding, temporary illness, or travel, although they constitute valid excuses for not fasting, they do not constitute grounds for paying fidyah. Such individuals remain obliged to make up their fasts at the first suitable opportunity.
Even if a person refrains from fasting on the belief—either personally or based upon a doctor’s statement—that there remains no possibility of recovery, and instead pays fidyah, but later recovers, the fidyah previously given will not remove the obligation of fasting from him, and he will still be required to make up the fasts that remained outstanding.