ضَرَبَ is a past-tense verb.
غُلَامُهُ is a nominative subject: “servant” is in the construct state and the attached pronoun completes it.
زَيْدًا is an accusative object.
If the pronoun means Zayd, the subject contains a pronoun whose referent comes afterward as an object. The referent is posterior in both wording and relevant syntactic ranking.
ضَرَبَ زَيْدًا غُلَامُهُ
His servant struck Zayd.
In the rearranged construction, Zayd is explicitly mentioned before the pronoun. The two word orders therefore differ in the relevant grammatical respect.