حُرُّ الْكَلَامِ كَلَامُ الْحُرِّ
Noble, free speech is the speech of a free person.
In “ḥurr al-kalām, kalām al-ḥurr,” the head and dependent expressions exchange syntactic positions. The first phrase and the second phrase remain meaningful despite the reversal.
The elegance is more than a mechanical flip: the altered relations contribute a connected second thought. If the inversion destroys intelligibility, it does not achieve the intended rhetorical quality.