Wa alaikum assalam, the prophet SAW said:
“When a slave falls ill or travels, then he will get a reward similar to that he gets for good deeds practiced at home when in good health” [Bukhari]
So based on this merciful ‘Hadith’, we expect the person who faithfully fasted in the past before qualifying for exemption, to be equally rewarded for fasting when they are unable to fulfill such acts of worship. Having said so, and to be on the safe side, it is “highly” recommended for any person who is unable to fast Ramadan, to break the fast of any Muslim on daily basis during the month of Ramadan, for the prophet SAW also said:
“He who provides a fasting person something with which to break his fast, will earn the same reward as the one who was observing the fast, without diminishing in any way the reward of the latter” [Tirmidhi]
This way, they are guaranteed the reward inshaa Allah.
And Allah knows best..