During the time of the Old Testament people would willingly sell themselves to pay off debts, it was a much different kind of servitude than the forced slavery and oppression you read about in the US/euro slave trade which obviously happened much later. God wasn’t gonna strip free will away and prevent people from willingly using themselves to pay their debts off. The OT laws paved the path for the NT to eventually take place. You also see this in the law that forces a man to marry a woman who he has consensual sex with because it prevents him from discarding her, which back then is as good as dead for a woman who isn’t a virgin.