The Song Dynasty permanent slavery for the lack of a better word was banned and the longest you could "own" someone was 2 years and they had to be paid.

During the Ming Dynasty slavery had already been banned once or slaves had all be released but during its later periods arose again and influence from the Mongol Yuan Dynasty.

During the 18 century the Yong Zheng Emperor had banned the owning of slaves but after a while people went back at it again.