Through one that sinned (di' henos hamartēsantos). “Through one having sinned.” That is Adam. Another contrast, difference in source (ek).
Of one (ex henos). Supply paraptōmatos, Adam’s one transgression.
Of many trespasses (ek pollōn paraptōmatōn). The gift by Christ grew out of manifold sins by Adam’s progeny.
Justification (dikaiōma). Act of righteousness, result, ordinance (Rom 1:32; Rom 2:26; Rom 8:4), righteous deed (Rom 5:18), verdict as here (acquittal).