Answer
✅ Answer: Please pay attention to the following verse and narrations:
قال اللَّه تعالي : أَيَّامًا مَّعْدُودَاتٍ فَمَن كَانَ مِنكُم مَّرِيضًا أَوْ عَلَى سَفَرٍ فَعِدَّةٌ مِّنْ أَيَّامٍ أُخَرَ...
📖Surah Al-Baqarah, Verse 184
Fasting is for a limited number of days, and whoever among you is sick or on a journey, then (he should fast) a number of other days.
1 عَنِ ابْنِ أَبِی عُمَیْرٍ عَنْ بَعْضِ أَصْحَابِنا عَنْ أَبِی عَبْدِ اللَّهِ قَالَ سَمِعْتُهُ یَقُولُ قَالَ رَسُولُ اللَّهِ إِنَّ اللَّهَ عَزَّ وَ جَلَّ تَصَدَّقَ عَلَی مَرْضَی أُمَّتِی وَ مُسَافِرِیهَا بِالتَّقْصِیرِ وَ الْإِفْطَارِ أَ یَسُرُّ أَحَدَکُمْ إِذَا تَصَدَّقَ بِصَدَقَةٍ أَنْ تُرَدَّ عَلَیْهِ.
📚 Al-Kafi, Vol. 4, p. 127
One of the companions said: 'I heard Imam Sadiq (peace be upon him) say: 'The Messenger of Allah (peace be upon him) said: 'Indeed, Allah, the Mighty and Glorious, has given a charity to the sick among my nation and their travelers by allowing them to shorten their prayers and to break their fast! Does one of you like it when he gives a charity, for it to be returned to him?!''
2 عَلِیُّ بْنُ جَعْفَرٍ فِی کِتَابِهِ عَنْ أَخِیهِ مُوسَی بْنِ جَعْفَرٍ قَالَ: سَأَلْتُهُ عَنْ حَدِّ مَا یَجِبُ عَلَی الْمَرِیضِ تَرْکُ الصَّوْمِ قَالَ کُلُّ شَیْءٍ مِنَ الْمَرَضِ أَضَرَّ بِهِ الصَّوْمُ فَهُوَ یَسَعُهُ تَرْکُ الصَّوْمِ.
📚 Masa'il Ali ibn Ja'far, p. 171, p. 295.
Ali ibn Ja'far said: 'I asked Musa ibn Ja'far (peace be upon him) about the limit of an illness for which breaking the fast is obligatory.' The Imam said: 'Any illness for which fasting is harmful, then in that case, there is permission and leeway to not fast.'
3 عَنِ الْقَاسِمِ بْنِ مُحَمَّدٍ عَنْ سُلَیْمَانَ بْنِ دَاوُدَ عَنْ سُفْیَانَ بْنِ عُیَیْنَةَ عَنِ الزُّهْرِیِّ عَنْ عَلِیِّ بْنِ الْحُسَیْنِ فِی حَدِیثٍ قَالَ: وَ أَمَّا صَوْمُ السَّفَرِ وَ الْمَرَضِ فَإِنَّ الْعَامَّةَ قَدِ اخْتَلَفَتْ فِی ذَلِکَ فَقَالَ قَوْمٌ یَصُومُ وَ قَالَ آخَرُونَ لَا یَصُومُ وَ قَالَ قَوْمٌ إِنْ شَاءَ صَامَ وَ إِنْ شَاءَ أَفْطَرَ وَ أَمَّا نَحْنُ فَنَقُولُ یُفْطِرُ فِی الْحَالَیْنِ جَمِیعاً فَإِنْ صَامَ فِی حَالِ السَّفَرِ أَوْ فِی حَالِ الْمَرَضِ فَعَلَیْهِ الْقَضَاءُ فَإِنَّ اللَّهَ عَزَّ وَ جَلَّ یَقُولُ فَمَنْ کانَ مِنْکُمْ مَرِیضاً أَوْ عَلی سَفَرٍ فَعِدَّةٌ مِنْ أَیَّامٍ أُخَرَ فَهَذَا تَفْسِیرُ الصِّیَامِ.
📚 Al-Kafi, Vol. 4, p. 86
Imam Sajjad (peace be upon him) said in a hadith: 'As for fasting during travel and illness, the general public has differed on this matter: one group said such a person should fast, another group said he should not fast, and another group said: 'If he wishes, he can fast, and if he wishes, he can break the fast!'
But we say that in both cases, he should break the fast and not fast! So if he fasts while traveling or in a state of illness, making up the fast is still his responsibility! Because Allah, the Mighty and Glorious, says (in Surah Al-Baqarah, verse 184): 'Whoever among you is sick or on a journey (during Ramadan), then (he should fast) a number of other days!'
✅ Based on the above verse and narrations, a person who has an illness during Ramadan for which fasting would be harmful is allowed and even obligated to break his fast. Such a person, if he regains the ability to fast after Ramadan and before the next Ramadan, is obligated to make up his fasts.
Some people who have diabetes are not able to fast when Ramadan is in the summer, but later, they regain the ability and opportunity to make up the fasts during the short days of winter. According to the above narrations, such people are obligated to make up their fasts.
In case of delay and negligence in making up the fasts until before the next Ramadan, in addition to making up the fasts, the person is also obligated to give a 'mudd' of food for each day of fasting as charity, the narrations for which you can see in the following link:
اللَّهُمَّ صَلِّ عَلَی مُحَمَّدٍ وَ آلِ مُحَمَّدٍ وَ عَجِّلْ لِوَلِیِّکَ الْفَرَجَ وَ الْعَافِیَةَ وَ النَّصْر