There are roughly 4,000,000 slaves in the USA in 1860, if you throw in the free blacks lets call it 4,250,000 African-Americans in the USA in 1860. Let's guess you can put 250 on an average passenger ship of the day in acceptable conditions - not like the slave ships that brought them. That means A LOT of trips from the US to Africa, and the cost of the round trip has to be paid for with cargo only going one way as there is not enough imports from Africa to be shipped back from Africa to the USA. This works out to 170,000 round trips. Even at 500/ship (truly brutal conditions) this is 85,000 trips. The cost of a third class trip to the USA in the 1880s and beyond on a steamship immigrant class was about $30. Assume the cost the same for an 1865 trip USA to Africa, it is a reasonable estimate. This means the shipping cost for these folks is $120,000,000. Now add the costs of getting these folks to ports of embarkation, housing and feeding them until they get aboard ship for a process that will take several years. Wow, now you are talking about real money. BTW just the shipping is $3,273,600,000 (roughly) in 2018 dollars. Don't forget the cost of the needs to settle in Africa (tools, seed, food, etc).
OK Skippy the ASB has dropped about 10 billion dollars (today's value) on you. OTL with the failure of reconstruction, Jim Crow etc, the number of African Americans who left the USA for anywhere after slavery ended, let alone those who went to Africa was basically a rounding error in the census. Exactly how are you going to get 4,250,000 people to do this voluntarily? You can't, not unless you have a military willing to do what the Germans did to the Jews.