Contrary to popular belief that Muslims freed slaves, in fact Arabs were actively involved in Slave trade, even today they are secretly conducting slave trade in Sudan and Nigeria and after them came the white people who slaved Africans.
The ACS (American Colonization Society) was the most actively involve organization in bringing a halt to the slave trade. The ACS organization was established in 1816, to sponsor the settlement of "free persons of color" in Africa. The ACS concerns were on a humanitarian, social and religious basis. In association with church and other antislavery groups the ACS lobbied effectively for legislation to halt the slave trade. Although the ACS did not receive direct financial aid from the United States government, it was still appointed as the custodian of recaptives under its protection by the government.
It was the Christians who in fact freed the slaves not Muslims!
Muhammad's (prophet) Slaves
Somebody Said to me that Muhammad (prophet) never had any slaves, what an ignorant person he is who doesn't knows who he follows, well here are the slaves that the prophet kept:
Muhammad's (prophet) Male Slaves
Yakan Abu Sharh, Aflah, Ubayd, Dhakwan, Tahman, Mirwan, Hunayn, Sanad, Fadala Yamamin, Anjasha al-Hadi, Mad' am, Karkara, Abu Rafi, Thawban, Ab Kabsha, Salih, Rabah, Yara Nubyan, Fadila, Waqid, Mabur, Abu Waqid, Kasam, Abu 'Ayb, Abu Muwayhiba, Zayd Ibn Haritha, and also a black slave called Mahran who was renamed by Muhammad as Safina ('Ship').
This makes 26 male slaves
Muhammad's (prophet) Maid Slaves:
Salma Um Rafi', Maymuna daughter of Abu Asib, Maymuna daughter of Sa'd, Khadra, Radwam, Razina, Um Damira, Rayhana, Mary the Coptic, in addition to two other maid slaves, one of them given to him as a present by his cousin, Zaynab, and the other one captured in a war.
This makes 11 maid slaves, it is well known that he had slept with Mariam the Copt and she even bore him a son named Ibrahim prior to Muhammad marrying her.
Ali Ibn Abi-Talib mentioned that a man had killed his slave and Muhammad (prophet) scourged him only; he did not kill him. It was related on the authority of Muhammad (prophet) that he said that a Muslim should not be killed for a non Muslim, nor a freeman for a slave; also because Abu Bakr and Uma Ibn al-Khattab did not kill a freeman for a slave.