1. The Prophet (PBUH) was a merchant that sold cloths and etc. He wasn't rich enough to put food on his plate at times let alone able to buy slaves.
2. Islam banned Slavery by regulating it in stages like Alcohol. First it was restricted, then selling was banned etc. until a final ban.
3. Just because a muslim empire did something bad doesn't mean it represents Islam because it has title Islamic in it?
Narrated Abu Huraira: Allah's Apostle said, "There is no Zakat either on a horse or
a slave belonging to a Muslim"
Sahih Bukhari 2:24:542
Abu Sa'id al-Khudri reported that at the Battle of Hunain Allah's Messenger sent an army to Autas and encountered the enemy and fought with them. Having overcome them and taken them captives, the Companions of Allah's Messenger seemed to refrain from having intercourse with captive women because of their husbands being polytheists. Then Allah, Most High, sent down regarding that:" And women already married, except those whom your right hands possess (Quran 4:. 24)" (i. e. they were lawful for them when their 'Idda period came to an end).
Sahih Muslim 8:3432
Narrated 'Abdul 'Aziz: Anas said, 'When Allah's Apostle invaded Khaibar, we offered the Fajr prayer there yearly in the morning) when it was still dark. The Prophet rode and Abu Talha rode too and I was riding behind Abu Talha. The Prophet passed through the lane of Khaibar quickly and my knee was touching the thigh of the Prophet . He uncovered his thigh and I saw the whiteness of the thigh of the Prophet. When he entered the town, he said, 'Allahu Akbar! Khaibar is ruined. Whenever we approach near a (hostile) nation (to fight) then evil will be the morning of those who have been warned.' He repeated this thrice. The people came out for their jobs and some of them said, 'Muhammad (has come).' (Some of our companions added, "With his army.") We conquered Khaibar,
took the captives, and the booty was collected. Dihya came and said, '
O Allah's Prophet! Give me a slave girl from the captives.' The Prophet said, 'Go and take any slave girl.' He took Safiya bint Huyai. A man came to the Prophet and said, 'O Allah's Apostles! You gave Safiya bint Huyai to Dihya and she is the chief mistress of the tribes of Quraiza and An-Nadir and she befits none but you.' So the Prophet said, 'Bring him along with her.' So Dihya came with her and when the Prophet saw her, he said to Dihya, '
Take any slave girl other than her from the captives.' Anas added: The Prophet then manumitted her and married her..."
Sahih Bukhari 1:8:367
So Muhammad began seizing their herds and their property bit by bit. He conquered home by home. The Messenger took some people captive, including Safiyah and her two cousins. The Prophet chose Safiyah for himself.
Al-Tabari, Vol. 8, p. 116, See also: Ishaq:511
Jabir (Allah be pleased with him) reported: There came a slave and pledg- ed allegiance to Allah's Apostle (may peace be upon him) on migration; he (the Holy Prophet) did not know that he was a slave. Then there came his master and demanded him back, whereupon Allah's Apostle (may peace be upon him) said:
Sell him to me. And he bought him for two black slaves, and he did not afterwards take allegiance from anyone until he had asked him whether he was a slave (or a free man)
Sahih Muslim 10:3901
It seems you have little to no understanding of your own religion. Slavery, as shown above, is clearly practiced by your beloved prophet who was apparently the best example for every time.
Also, provide evidence from Quran, Hadith and Serah (biography of the prophet) that directly state that Islam came to outlaw slavery. Sure, Islam did demand the good treatment of slaves on many occasions but that is not evidence it came to outlaw slavery and the fact that slavery was outlawed in the Muslim world 1300 years AFTER the advent of Islam clearly contradicts this. Secondly, you cannot include anything into the religion after it was claimed to be completed and thus your argument is void.