In The Known World, what is the quote where Augustus is disgusted with Henry for owning slaves?

I believe the quote you are looking for is in Chapter 4 (page 138; this is the 2003 edition of the book), where Henry finally admits to his parents that he owns slaves.

Of all the human beins on God's earth, I never once thought the first slaveowner I would tell to leave my place would be my own child. I never thought it would be you. Why did we ever buy you offa Robbins...

I believe the quote you are looking for is in Chapter 4 (page 138; this is the 2003 edition of the book), where Henry finally admits to his parents that he owns slaves.

Of all the human beins on God's earth, I never once thought the first slaveowner I would tell to leave my place would be my own child. I never thought it would be you. Why did we ever buy you offa Robbins if you gon do this? Why trouble ourselves with you bein free, Henry? You could not have hurt me more if you had cut off my arms and my legs.

As a black man, Henry must purchase his own slaves through William Robbins, a white plantation owner.

Using Henry's money, Robbins did all Henry's purchases of slaves before 1850 when a delegate from Manchester had the law changed. Most white men knew that when they sold a slave to Robbins, they were really selling to Henry Townsend (page 50).

Mildred and Augustus are deeply grieved at Henry's confession. Mildred accuses her son of doing things the "same old bad way." She has always taught her son that a man who was once a slave must never own one himself. It would be like going back to Egypt "after God done took you outa there." Meanwhile, Augustus is so furious that he beats Henry with a stick. Eventually, the altercation ends with Henry leaving his parents' house, as his mother looks on sadly.