All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If a person is sick with an illness that is expected to be cured, then he is obliged to make up after Ramadan whatever days he had missed in Ramadan.
Therefore, if it is expected that you will be cured from your illness, then it is not sufficient for you to pay a ransom now; rather, you must make those days up after you are cured even if it is after a long time.
However, you are not obliged to pay a ransom due to the delay in making up those days because of being unable due to sickness.
Asna al-Mataalib, authored by Zakariyyah al-Ansaari reads:
“If he delays making up (for fasting) due to an excuse, such as if he was a continuous traveler, or in continuous sickness, or a pregnant woman, or a breastfeeding woman, until the next year, then he/she is not required of anything because of the delay. This is because delaying the current fasting [i.e. in Ramadan] because of a sound excuse is permissible, so it is with greater reason that delaying making it up [the fasting] is also permissible.”
Allah knows best.