It is permissible for a diabetic patient to skip fasts in Ramadan if they have become weak to the point where it is difficult for them to fast due to their medical condition.
As for the compensation for these missed fasts, as long as there is hope for the patient to recover and regain the ability and strength to fast, these fasts cannot be compensated for simply by giving fidyah. Once the patient feels healthy and strong enough to fast, they must perform qaza for the missed fasts.
Giving fidyah as compensation will only be permissible if there is no hope for the patient to regain their health and the ability to fast, and it is unlikely that they will recover from their medical condition during their lifetime.
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دلائل:
الھندیة: (207/1، ط: رشیدیة)
المریض اذا خاف علی نفسہ التلف او ذھاب عضو یفطر بالا جماع وان خاف زیادۃ العلۃ وامتداوہ فکذلک عندنا وعلیہ القضاء اذا فطرکذا فی المحیط ثم معرفۃ ذالک باجتھاد المریض والا جتھا د غیر مجردا لو ھم بل ھو غلبۃ ظن عن امارۃ او تجربۃ اوبا خبار طبیب مسلم الخ۔ غیر ظاھر الفسق کذافی فتح القدیر ۔
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.