The Mali Empire peaked around 150 years before Christopher Columbus "discovers" the Americas, and it finally collapses in 1599, finished off by Morocco. The Transatlantic Slave Trade doesn't really kick into gear until the mid-17th centuries, after the Mali Empire is no longer around.

If you wanted a better example, you might have picked, say, Benin. That you use Mali as an example of an empire screwed by Europeans in Africa suggests to me a general poverty of knowledge about any pre-colonial African history.