Be careful here. The presence of a specific trait, like sickle-cell, doesn't necessarily mean that the population has evolved it, and the presence of very, very small numbers of the sickle-cell gene in Italians doesn't necessarily mean that Italians are adapted to be resistant against malaria. The Northern Africans who invaded as part of the Muslim conquests had interbred at least a little with Sub-Saharan Africans who carry the sickle-cell gene, and thus by interbreeding with Italians would carry the gene in very small amounts to the Italian population. So the presence of the gene could simply be due to genetic diffusion and not adaptation.
True, but the basic point remains the same, namely that thay have African genes in addition to SC that may help resist malaria like it did for some of their ancestors. Of course, it is difficult to measure how helpful that would be without a specific genetic trait, so I hoped to use SC as a means of tracking trends of malaria resistance. So in other words, 1 in 10 Africans have SC, but all of them stand a better chance of surviving malaria than the average European, so for the Italians 1-2 in 100 have SC, but a considerably greater percentage have built up immunity to malaria from their African ancestors.
Of course, they would still only be the best of a bad economic situation compared to the slave trade, but if slavery is barred I doubt that the Europeans will go to Africa to hire them as wage workers with rights, so best of a bad situation might be the best the plantation crops can get with this limitation.