The Prophet Muhammad, peace be upon him, had heard of and seen the torture of Bilal, which pained him extremely to learn that a true believer suffered in such a manner. So, He told Abu Bakr to purchase him from Ummayya and set Bilal free.
In the same way, the Prophet, PBUH, had many slaves set free. His teachings insisted that slaves be treated fairly and kindly and he set out an example of his own conducts.