Matthew 8:9

καὶ γάρ, ‘for indeed.’ καὶ connects the reason why Christ should not enter more closely with the facts of the centurion’s position.

ὑπὸ ἐξουσίαν, ‘under authority,’ e.g. that of the χιλίαρχος or tribunus militum: cp. Acts 21:32, ὅς (χιλίαρχος) παραλαβὼν στρατιώτας καὶ ἑκατοντάρχας.

τούτῳ [στρατιώτῃ] … τῷ δούλῳ μου. Observe a distinction in the centurion’s orders, his soldiers come and go, i.e. march when he bids them. His slave he orders to do this, i.e. perform any servile work. In the household of the centurion Cornelius we find as here οἰκέται and στρατιῶται (Acts 10:7).

Mark this as the first contact of Jesus with slavery. With such relations between master and slave as these slavery would soon pass away.
It was no express enactment of Christ, but the Spirit of Christ, which this centurion had caught, that abolished slavery.