Not really true. The Jews had lots of slaves, as did Abraham and the other patriarchs. All ancient peoples practiced slavery and took it completely for granted. Even the NT accepts it, to the extent that Paul counseled a runaway slave to voluntarily return to his master.
The OT did spell out rules for how slaves were to be treated, and all Jewish slaves were to be freed every seventh year. This meant that Jewish slavery was more like indentured servitude for a specific period than the lifetime chattel slavery we think of.
Apparently this provision was not always followed, and it doesn't seem to have ever applied to non-Jewish slaves.
The concept that there should be NO slaves did, however, evolve out of the "all men are brothers" idea created by the Jews.
I think Paul, for all the bashing he takes, planted the revolutionary seed that, if a master and a slave are both Christians, then doesn't that drastically change the relationship between them?
As for the slave he sent back, I believe he begs the owner to free him because he had become so dear to Paul.