In 1791 John Asher was taxable on two slaves over 16 years of age in Culpeper County.
In 1796 William Asher was a "ma" taxable in the same district, so he may have been emancipated about that time. The record would be in the wills or deeds which you can borrow from the Library of Virginia on interlibrary loan to the nearest library to you with a microfilm reader (usually your county library).
I have the tax lists from 1803-1823 on order and will let you know if they add anything.
You might also order the Personal Property tax lists around 1835 since many counties made separate lists of free African Americans around that time.
Paul