It is striking to see what percentage of American capital attributed to slavery in the 18th and 19th centuries (the striped section in the chart above). In the late 18th century only agricultural land counted for more, and there slavery contributed to that too.
The American Civil War and the emancipation of those bound in slavery destroyed all that capital, and that was great and necessary. While it is wrong to consider slavery only in terms of money, it is impossible to talk about slavery in the United States without considering its relationship to the economy and capital. The capital that derived from slavery was massive.
In the U.K. the abolition of slavery resulted in the government providing capital back to the slave owners. It was a terrible omission that neither the U.K. nor the U.S. provided capital to the freed slaves. There are those, like Ta-Nehisi Coates, who argue that such capital in the form of reparation is due. Based on the chart above, a case could be made that it would be a tremendous amount of money.
(Chart above taken from “Capital in the Twenty-First Century” by Thomas Piketty)