As the narrator drives Mr. Norton off campus, Norton wants to stop at one of the poor cabins they see. He marvels when the narrator tells him it was built to house slaves but also seems shocked at the poverty. Then, the narrator tells of the scandal surrounding the man who lives in the cabin with his family. Jim Trueblood slept with and impregnated his daughter and his wife.
Norton is appalled but seeks out Trueblood, who confirms the story and seems calm about it. Norton responds with:
"You did and are unharmed!" he shouted, his blue eyes blazing into the black face with something like envy and indignation.
The word envy may be seen as a clue as to why he gives Trueblood the hundred dollars (about a thousand by today's standards).
Trueblood tells of being sent to Sheriff Barbour. Trueblood says,
I tole him and he called in some more men and they made me tell it again. They wanted to hear about the gal lots of times.
Trueblood then goes on to flesh out to Norton the story of what happened in pornographic detail, offering what we might call "too much information." The narrator is embarrassed, but Norton is both horrified and spellbound.
Trueblood says that while white people have been protecting him against the wrath of the Black community and giving him things ever since the sex act with his daughter, they wouldn't help him before the incest.
It's not made perfectly explicit why the Norton gives Trueblood the hundred dollars. On the surface, it is to help out a poor Black man who had to sleep in a bed with his wife and daughter in the winter to keep from freezing. To some extent, Norton may have truly felt he was helping out a family in need. But readers may be warranted in looking for and interpreting a different reason.
Because of the mention of Norton's "envy" and fascination at Trueblood's story—not to mention the fact that Norton himself at one point describes his daughter as "a being more rare, more beautiful, purer, more perfect and more delicate than the wildest dream of a poet"—it may be argued that Norton is rewarding a story of incest that fulfills his own sexual desires.