Negroes working in stores often acquired a fair education by assisting clerks.

Some slaves were clerks themselves.

Under the observation of E.P.Burke came the notable case of a young man belonging to one of the best families of Savannah.

He could read, write, cipher, and transact business so intelligently that his master often committed important trusts to his care.[1] B.K.
Bruce, while still a slave, educated himself when he was working at the printer's trade in Brunswick, Missouri.

Even farther south where slavery assumed its worst form, we find that this condition obtained.
Addressing to the New Orleans _Commercial Bulletin_ a letter on African colonization, John McDonogh stated that the work imposed on his slaves required some education for which he willingly provided.