A slave contract does not legitimize slavery
Let's say there were a document that laid out the ground rules for what a slave master was allowed to do, and NOT do, to his slaves. It might state that a slave master must give his slaves adequate food. Maybe insist that he not beat his slaves too much. It might set the rules for selling the slaves in a "humane" way. It might even lay out a list of rights a slave has, as long as that slave doesn't try to escape or disobey. It would probably mention some "obligations" the slave "owes" his master; things like loyalty, obedience, and hard work.
The problem with such a document isn't that it is flawed in its rules, but that it treats slavery as a legitimate human endeavor. It isn't, and no document can ever make it so.
There is another illegitimate human endeavor, based upon theft and murder, that has a document that gives it the illusion of legitimacy to some people. A government is supposedly legitimized by the US Constitution, much to the delight of statists of all sorts. Others of us can't justify the inexcusable no matter who signed what a couple hundred years ago. And just because other governments might be worse is no reason to stick with a coercive state of any degree.
Just as I am not qualified or inclined to pontificate upon the proper treatment of slaves, other than unconditional freedom, I am not qualified or inclined to decide what might be the proper form of government, other than self-government. No document can ever make wrong right. We need to banish the idea that it can before we can move forward. Any other conclusion just doesn't stand up to scrutiny.