Fasting during the month of Ramadan is one of the five pillars of Islam that are of paramount significance. The Prophet (peace and blessings be upon him) said, “Islam is built upon five pillars: testifying that there is no god except Allah and that Muhammad is the Messenger of Allah, performing Prayer, paying the zakah, making the pilgrimage to the Sacred House (Hajj), and fasting the month of Ramadan” (Al-Bukhari and Muslim). Scholars have unanimously agreed that a patient is allowed not to fast, as Almighty Allah says: (Whosoever of you is sick or on a journey, (let him fast the same) number of other days. Allah desireth for you ease; He desireth not hardship for you) (Al-Baqarah 2:185).
Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto,Ontario, Canada, states the following: If you have been advised by your doctor not to fast, then you should not fast, but you ought to give a fidyah (compensation) by parting with the equivalent of the cost of a meal to the poor for each day of fast you have missed—if your sickness is permanent and there is no hope for recovery. If, however, your condition is temporary and you have reasonable expectation to recover from your sickness, then you need not offer fidyah (compensation); rather you only need to make up for the days you have missed later on when you are in a condition to do so. Allah says: ( Whosoever of you is sick or on a journey, (let him fast the same) number of other days) (Al-Baqarah 2:185).
If, on the other hand, you do not hope to recover from your sickness, then you should offer fidyah if you can afford to. But if you cannot afford to pay because of lack of means, then you are excused.
Having said this, I must still point out that, if a muslim’s financial situation does improve at all, they still ought to pay the debt you have incurred. The Prophet (peace and blessings be upon him) said, “Allah’s debt merits greater priority in terms of repayment” (Al-Bukhari).