I've heard about this, and I'd like to know more and the context of these events.
I know St Augustine was very much in favor of slavery. It'd be odd if the roman church was against it so early on.
Pope Eugene IV did not ban enslaving everyone. He only banned enslaving Christians, especially newly converted Christians.
Eugene IV was Pope from 1431 to 1447 (though there was a rival claimant to the title for some of his papacy).
Eugene was involved in conflict, military and political, throughout his pontificate. His greatest conflict was with the council of Basil (called by his predecessor) which he tried to dissolve, but which remained in being and opposed him, appointing an anti-pope against him. Eugene was also involved in wars in the Papal States. He was forced to flee Rome, and ran his papacy from Florence for 10 years.
Despite all these struggles, Eugene was able to attend to other business from time to time. Most notably, he effected union (though short lived) with the Eastern Orthodox Church, the Armenian Church, the Nestorian and Maronite Churches. He also declared and funded a crusade against Ottoman Turks (which failed in dismal defeat at the battle of Varna, leading eventually to the downfall of Constantinople and the end of the last remnants of the Eastern Roman Empire).
Amongst all of this, Eugene, managed to get involved in a quarrel between Spain and Portugal over the Canary Islands. Possession of these Islands was disputed between Portugal and Spain. Raiders from both countries were seizing slaves from amongst the Canary Islanders. These Islanders had begun converting in large numbers to Christianity by the 1430s. In 1434, Eugene issued the Papal Bull "Creator Omnium", which excommunicated anyone who enslaved newly converted Christians, and insisted that these slaves should be freed.
The Portuguese (King Edward of Portugal) asked for clarification around this bull and were reassured by the Pope that unconverted parts of the Canary Islands could still be conquered and the people enslaved.
In 1442, Pope Eugene, in response to a request from Prince Henry (the Navigator) of Portugal, issued the Bull "Illius Qui" which declared Portuguese expeditions down the coast of Africa to be a crusade, and allowed the taking of slaves in that endeavor.
Thus, Pope Eugene limited his ban on enslaving people to enslaving Christians, and especially newly converted Christians. He did not ban, and might be said to have encouraged the African slave trade, which was just beginning to emerge during his Papacy.
Sources: http://www.newadvent.org/cathen/05601a.htm