جَاءَ التَّاجِرُ وَهُوَ يَحْمِلُ مِيزَانَهُ.
The merchant arrived while carrying his scale.
Huwa is the subject of a nominal clause whose predicate is the verbal clause yaḥmilu mīzānahu. The whole nominal clause functions circumstantially. The wāw marks circumstance, and the pronoun huwa refers back to the merchant.
WordingNatural analysis
jāʾa al-tājiru yaḥmilu mīzānahuAffirmative imperfect verbal ḥāl
jāʾa al-tājiru wa-huwa yaḥmilu mīzānahuNominal ḥāl with wāw and pronoun
jāʾa al-tājiru wa-ḥamala mīzānahuPotential coordination of two actions
The first two directly express concurrence; the third can present a separate subsequent act.