❖ Question:
If someone still has obligatory (qaḍāʾ) fasts remaining, is it permissible for him to observe voluntary (nafl) fasts?
Source: Fatāwā Amunpuri by Shaykh Ghulam Mustafa Zaheer Amunpuri
✿ Answer:
Yes, even if some obligatory make-up fasts remain, it is still permissible to observe voluntary fasts.
❀ Statement of Imām al-Nawawī رحمه الله:
يَجُوزُ صِيَامُ النَّفْلِ مَعَ بَقَاءِ قَضَاءِ رَمَضَانَ
"It is permissible to fast voluntarily even if the make-up fasts of Ramadan are still pending."
(al-Majmūʿ Sharḥ al-Muhadhdhab: 6/375)

① Voluntary fasts may be observed even if some qaḍāʾ fasts are outstanding.
② However, priority should be given to fulfilling obligatory fasts first.