Slavery in the Bible was different than in modern times.  The issue of property deals with what is produced, not that the person was actual property.  We know this because property was to be returned to the owner. But, the Bible prohibited the extradition of slaves and granted them asylum (Deut. 23:16-17).  Therefore, they were not property in the sense of chattel slavery.

“And if a man strikes his male or female slave with a rod and he dies at his hand, he shall be punished. 21 If, however, he survives a day or two, no vengeance shall be taken; for he is his property,” (Exodus 21:20-21).

What are we to make of the disciplinary statement of Scripture regarding a slave?

“Again, when property is spoken of, oxen, sheep, &c., the term owner is always used, master never; when servants and masters are spoken of, master is always used, owner never.”¹ Allen, Isaac. Is Slavery Sanctioned by the Bible? (p. 13). Kindle Edition.

God permitted slavery to exist in both the Old and New Testament times. But this does not mean that slavery was a God-ordained system. Slavery was an invention of fallen man – not of God. Nevertheless, God allowed it to exist the way He allows other things to exist that He does not approve of murder, lying, rape, theft, etc.

God also works within the system of fallen man and makes allowances for the freedom and failures of mankind within that system. We see this, for example, in Jesus saying that God allowed divorce because of the hardness of peoples’ hearts (Matt. 19:8). The fact is, people are sinners and do things contrary to the will of God. But, even though people have murdered, lied, raped, and stolen, God has still used people who’ve committed these sins to accomplish His divine will. Moses murdered an Egyptian but was used by God to deliver Israel. David committed adultery but was promised to have the Messiah descend from his seed. This is proof that though God desires that people not do much of what they do, He permits them their freedom yet uses the system and the people according to His divine will.

In the case of a slave being property, that is simply the way things were done back then. As I said, God worked within the fallen system of man and put limits and guidelines concerning the treatment of slaves.