I'm curious how Colson arrives at this remarkable conclusion. The Bible, particularly the OT but also the NT, takes slavery very much for granted as a part of life.
It doesn't order people to practice it, but it never once implies that they shouldn't.
The concepts of human dignity and equality that eventually led to the end of slavery are embedded in the Bible, but it seems pretty clear that those living at the time didn't make the connection.
The Bible contains prescriptions for divorce as well, but God also says He hates divorce and Jesus said in one of the Gospels that a man who divorces his wife for any reason but adultery is himself an adulterer if he remarries. But never once does it say, "Never, ever get divorced."
By your standard, God sanctions divorce.