Because the Israelites are My servants, whom I brought out of the land of Egypt, they are not to be sold as slaves.
42. For they are my servants. God here declares that His own right is invaded when those, whom He claims as His property, are taken into subjection by another; for He says that He acquired the people as His own when He redeemed them from Egypt. Whence He infers that His right is violated if any should usurp perpetual dominion over a Hebrew.
We have here the laws concerning servitude, designed to preserve the honour of the Jewish nation as a free people, and rescued by a divine power out of the house of bondage, into the glorious liberty of God's sons, his first-born. Now the law is, I. That a native Israelite should never be made a bondman for perpetuity.
Commenting on Leviticus 25:39-55
For they are my servants, which I brought forth out of the land of Egypt,.... The Lord redeemed them out of Egypt, made a purchase of them, and had a prior right unto them, and being his servants first, they cannot be the servants of others; his right unto them as such antecedes and prevents any other claim upon them: they shall not be sold...