“The most solid data we found was published in an article in the Root by Henry Louis Gates Jr., a Harvard University historian. Gates cited research by Carter G. Woodson, an African-American historian who died in 1950. He found that in 1830, a total of ‘3,776 free Negroes owned 12,907 slaves.’”
“It includes ‘many “owned” family members whom they had purchased to become free,’ said Eric Foner, a Columbia University historian and the author of such books as The Fiery Trial: Abraham Lincoln and American Slavery. ‘You could not free a slave in most southern states without sending them out of the state.’”
The story that I remember reading was something about a Freeman Black who was in a posse after escaped slaves in the Midwest and the local sheriff confronted the Posse and escorted them to the bridge for them to leave the state.
The data would come from census records. Because of the three-fifths rule, the slavery status of a black person was significant.