When Gilbert gave him a small plantation, he managed it with such astuteness […] that at the end of the time he could repay Gilbert a substantial part of the purchase price. (4.34)
We don't know yet just how big a role slavery will play in the story of Cass Mastern, but we do know that he's a slave owner. He didn't even have to ask for it. It was put in his lap by his brother, and Cass didn't even know he should refuse. But he learns.
"Oh she'd stay right here and look at me and tell, tell what she knows and I will not abide it!" (4.104)
This is Annabelle's response to Cass's question about why she sold Phebe into sexual bondage instead of freeing her. We think Annabelle is being irrational here. Phebe would never be believed over Annabelle, and she knows better than to try. Annabelle's action ultimately speaks to a deeper shame.
"Oh, I see, you are very concerned for the honor of a black coachman." (4.112)
Cass judges Annabelle's action all the more harshly because Phebe was married to a neighboring coachmen, and was thus separated from family. Annabelle first accuses Cass of wanting her sexually, and then pokes fun at his sentiment. Cass is a plantation owner, after all. He has no right to judge anybody in the matter of slavery. But, he is starting to see the ills of slavery.
Then he [the Frenchman] let his hand fall to the side, with the crop. "Open your mouth," he said to the girl.(4.133)
When Cass tries to find Phebe he goes to a place where young women are being "shown" to prospective buyers. They will be shipped to brothels to maximize their earning potential. The scene that precedes this quote is one of the most uncomfortable in the novel.
"For many cannot bear their eyes upon them, and enter into evil and cruel ways in their desperation." (4.151)
Cass is suggesting that the institution of slavery has so corrupted those who practice it, that its practitioners can no longer tamp down the shame. As talk of abolition becomes prominent, slave owners across America begin to see the evil of their ways.
In the end he repaid his debt, and set free his slaves. He had some notion of operating the plantation with the same force on a wage basis. (4.145)
In Cass's society, giving former slaves a salary was unheard of; this practice eventually leads to tragedy. The light bulb has gone of in Cass Mastern's head, but the problem is too big for him to solve alone.
So Cass put his free N****es on a boat bound upriver, and never heard of them again. (4.150)
Some of these former slaves may have reached freedom and managed to have a decent life, in spite of what Cass thinks. He thinks he sent them away because he "cannot bear their eyes upon him." In his case the people he freed look at him as a hero. His shame is in not being able to live up to his potential as a advocate of equal rights.
In any case, he laid aside the journal and entered upon a period of the Great Sleep. (4.166)
The piece of American history that Jack encounters in the Cass Mastern story is so shocking to Jack that he temporarily disassociates from the world. He can no longer understand where he is in space, time, or history.