If what you say is true, then slavery is only evil when society forbids it.<quoted text>
You're mixing the terms absolute morality and universal/objective morality.
You've yet to explain why different people appeal to different moral systems, if there is one universal moral system.
They're not transcendent; they're quite clearly based around culture, reason, and emotion (or something other things).
But if it condones it, it is not evil.
You understand the argument you are making don't you?