وَإِنْ حَلَقَ أَقَلَّ مِنَ الرُّبُعِ فَعَلَيْهِ صَدَقَةٌ
Translation: If he shaves less than one-quarter of the head, charity is due.
This is the contrasting threshold. If, in an illustrative case, one-fifth of the head was shaved, the specified quarter threshold is not reached, and the text calls for sadaqah.
Less than a quarter is not thereby permitted: the act remains prohibited. The passage classifies the compensation, rather than making a small violation lawful.