The correct term is actually prisoners of war, because that's the only time a "slave" can be captured. And if they repent and choose Islam, then they're usually freed. No one can enslave a Muslim or someone who wasn't captured during a war between Muslims & non-Muslims. The reward for freeing a slave is Jannah according to the Prophet (saw):
"And such of your slaves as seek a writing (of emancipation), give them such writing, if you find that there is good and honesty in them. And give them something (yourselves) out of the wealth of Allaah which He has bestowed upon you"
[al-Noor 24:33]
It was narrated that Abu Dharr (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said:
"They are your brothers whom Allaah has put under your authority, so if Allaah has put a person's brother under his authority, let him feed him from what he eats and clothe him from what he wears, and let him not overburden him with work, and if he does overburden him with work, then let him help him." Narrated by al-Bukhaari (6050).
It was narrated that Abu Hurayrah (may Allaah be pleased with him) said: I heard Abu'l-Qaasim (peace and blessings of Allaah be upon him) say:
"Whoever accuses his slave when he is innocent of what he says will be flogged on the Day of Resurrection, unless he is as he said." Narrated by al-Bukhaari (6858).
Ibn 'Umar (may Allaah be pleased with him) manumitted a slave of his, then he picked up a stick or something from the ground and said: There is no more reward in it than the equivalent of this, but I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say:
"Whoever slaps his slave or beats him, his expiation is to manumit him." Narrated by Muslim (1657).
Of course Muslims over the times like others abused their slaves. Even our forefathers had slaves in Somalia, which is pretty shameful what they did to them.
http://www.islam-qa.com/index.php?ref=9 ... xt=slavery