This recurrent theme in the Bible begs the question, why would God allow it and even give instructions concerning it. In fact, there is one law for the descendants of Abraham and another for the stranger (Exodus 12:49). None of this seems to marry up with our perception of slavery.
The forgotten commandment. I am not talking about some obscure reference in some pages of the Bible that haven’t seen the light of day since who knows when, but one of the 10 Commandments codified at Mt Sinai. We forget it, gloss over and yet God thought it important enough to be one of the Ten.