It is generally believed by Biblical scholars that the sabbath and Jubilee year provisions of the law were aspirations never actually put into practice. It is possible (male) slaves were indeed freed after seven years, but women slaves and their offspring were for life.
And non-Hebrew slaves were treated as permanent chattel, just like all the nations around them did.
It is true the first few years in America blacks were apparently treated as indentured servants, just like whites were. Based on Caribbean and Spanish precedent, this changed pretty quickly, with the adopting of civil (Roman) law precedence for dealing with slaves over English common law, which had no provision for chattel slavery.
For crying out loud, FRiend, are you going to blame the Bible because Jews didn't always follow its commandments?
What's up with that?
The Bible is quite clear and consistent in opposing "slavery" for God's people, and this is the concept you must, must grasp or you'll be forever confused.
Yes, it recognizes that slavery is sometimes necessary, even amongst Israelites, but it requires that slaves not be mistreated and must be released after seven years (see Exodus 21).
That requirement alone makes it not real "slavery", but an employment contract -- i.e., to pay off debts.
If you wish to see an actual example of how this worked in practice, try Jeremiah chapter 34.
The short of that story is that Jews knew perfectly well what God required, and when they really, really needed His help, were willing to do it.
By the way... if you love history as much as I do, then compare the story of Jeremiah 34, where Jews in extremis free their slaves to win God's favor, contrast to that of Carthaginians in precisely the same predicament, who offer up to their god (Baal?) what he wanted most -- they kill their children.
So don't tell me the Bible doesn't know right from wrong, FRiend.