فَإِنْ فَضَلَ مِنَ الطَّعَامِ أَقَلُّ مِنْ نِصْفِ صَاعٍ، فَهُوَ مُخَيَّرٌ: إِنْ شَاءَ تَصَدَّقَ بِهِ، وَإِنْ شَاءَ صَامَ عَنْهُ يَوْمًا كَامِلًا.
Translation: If a food remainder is smaller than half a saʿ, he has a choice: give that remainder as charity, or fast a full day for it.
A small remainder is not simply discarded. The text offers giving it away or observing one full fast for the remainder.
The phrase yawman kamilan means an entire fasting day, not a fractional or shortened fast.