Looking at historical documents is what historians and academics, do. They can determine an average time and labor spent to determine an average wage the slaves should've received to determine an average of what their descendants should get. Yes, really.
Most African Americans never had inherited wealth like the white middle class did and they didn't earn enough to pay into the general fund. Until 1960 they weren't even allowed to own a business ...or vote.
"African-Americans, before the 1960s, first by law and then by custom, were not really allowed to own businesses. They had very little access to credit. There was a very low artificial ceiling on the wealth that could be accumulated. Hence there was very little, if anything, that could be passed along to help their children get to college, to help their children buy their first homes, or as an inheritance when they die," said Shapiro....
A $95,000 question: why are whites five times richer than blacks in the US? | World news | The Guardian
"..Wealth passes down from generation to generation. The main reason African Americans are currently worse off than whites, according to Shapiro, is that today's African Americans inherited less wealth from their parents than today's whites did. It is not hard to see why: The generation of African Americans now passed away accumulated less wealth because discrimination in their day kept most of them poor and denied them opportunities other Americans enjoyed.
The disparity in wealth not only persists, it mushrooms. Without a cushion of inherited wealth, emergencies hit harder, and people who have no nest egg have to let opportunities pass by. Because of the wealth deficit, African Americans find themselves more vulnerable to shocks and less able to capitalize on breaks than whites with the same income. So the next generation will inherit less, too. The wealth gap will not close anytime soon...."
RACE - The Power of an Illusion . Background Readings | PBS
The racial wealth gap we hardly talk about: What happens in retirement - The Washington Post