He owned servants and not slaves. According to Islamic religion slaves can be only to God and not human beings. It is best explained by this Sheik :
http://www.youtube.com/watch?v=_ubpDRRkDis
There are a number of hadith that show that the Prophet treated servants well and expected others to do the same...
He will not enter Paradise who behaveth ill to his servants. The Companions said, 'O Apostle of God! have you not told us, that there will be a great many servants and orphans amongst your disciples?' He said, 'Yes; then be kind to them as to your own children, and give them to eat what you eat yourselves. The servants that say their prayers are your brothers.
Be kind to servants as to your own children...and those that say their prayers are your brethren.
They (servants) are your brothers, and Allah has put them under your command. So the one under whose hand Allah has put his brother, should feed him of what he eats, and give him dresses of what he wears, and should not ask him to do a thing beyond his capacity. And if at all he asks him to do a hard task, he should help him therein.'
'There are three categories of people against whom I shall myself be a plaintiff on the Day of Judgement. Of these three, one is he who enslaves a free man, then sells him and eats this money'.
al-Bukhari and Ibn Majjah
Narrated Abu Musa Al-Ash'ari: "The Prophet said, 'Give food to the hungry, pay a visit to the sick and release (set free) the one in captivity (by paying his ransom).'"
When Islam was reveled to Prophet Muhammad (peace be upon him), slavery was a worldwide common social phenomenon; it was much older than Islam. Slavery was deeply rooted in every society to the extent that it was impossible to imagine a civilized society without slaves. In spite of this social fact, Islam was the first religion to recognize slavery as a social illness that needed to be addressed. Since slavery was deeply rooted in the society, Islam did not abolish it at once. Rather, Islam treated slavery in the same manner it treated other social illnesses. Islam followed the same methodology of gradual elimination in dealing with this social disease as it did with other social illnesses, for example: the prohibition of alcohol in three steps.
From the early days of the message, Islam declared the equality of all human beings, including slaves; equality in origin, equality in values, equality in destiny. Under this declaration, for the first time ever, slaves became brothers and sisters of their masters in the Islamic Ummah (community). In al-Bukhari, it was reported that the Prophet said:
"Your servants are your brothers. Anyone who has servants should give them from what he eats and wears. He should not charge them with work beyond their capabilities. If you must set them to hard work, in any case, I advise you to help them."
Islam forbade the traditional practice of enslaving free individuals by capturing them and selling them into slavery. The Prophet (peace be upon him) said:
"There are three categories of people against whom I myself shall be a plaintiff on the Day of Judgment. Of those three, one is he who enslaves free men, then sells him and eats this money…"
Islam established rules, which leads to the eventual freedom of slaves. It declared the act of freeing slaves as a good deed, which is tremendously rewarded by Allah. Therefore, Muslims were encouraged to participate in freeing slaves. The freedom of slaves was also encoded in the legal system of Islam by requiring it as part of the penance for sins and as the punishment for criminal acts. Islam listed freeing slaves as one of the eight elements for which zakah (state collected alms money) could be used.
In addition, Islam established many other ways by which slaves could easily gain their freedom. For example, a baby born from a slave and her master is not only considered free, but also guaranteed the freedom of his mother. A second example is that if a slave was physically tortured by his master, he automatically becomes a free man! A third example is that slaves were allowed to buy their freedom from their masters, and are allowed to seek financial help to do so. Actually, you can find many other examples in the fiqh (Islamic law) books.
Eventually, through this systematic plan of gradually freeing the slaves and making them accepted in the society, the idea of eliminating slavery became powerful enough that after Prophet Muhammad (peace be upon him) died, the freed slaves remained free and not one of them was ever enslaved again. Also, within the times of the prophethood and the rightly guided caliphate (the first four caliphs) there were 320 slaves who were freed! See the book:Subul al-Salam of al-San'ani.
The final word is that although Islam did not abolish slavery, Islam was directing the Ummah toward it. In other words, it was Islam, which paved the path toward abolishing slavery.