During the story, it seems that slavery is being romanticized, that the old notions of "the happy slave" are being promoted. However, this is all in service of the twist ending, where the seemingly happy slave has been working a long con. Dick Owens has a grandiose plan to impress his true love: he will help a slave to freedom in Canada. However, his father the Colonel is suspicious, and gives him Grandison, who is the very stereotype of an "Uncle Remus" happy slave.
The colonel was beaming. This was true gratitude, and his feudal heart thrilled at such appreciative homage. What cold-blooded, heartless monsters they were who would break up this blissful relationship of kindly protection on the one hand, of wise subordination and loyal dependence on the other!
(Chesnutt, "The Passing of Grandison," facstaff.gpc.edu)
As Grandison steadfastly refuses to "escape," Dick finally gives up and escapes from Grandison instead, returning to the U.S. alone. Throughout the story, right up until Grandison returns of his own free will, it seems that the story is a classic piece of pro-slavery propaganda: the slaves are happy in their slavery, they need to be owned for their own good, they cannot live without mastery, etc. and so forth. However, Chesnutt's sneaky twist ending changes the entire context: Grandison has been playing the part of a happy slave for years, learning about the plantation's operation and the Colonel's movements, and the trip to Canada allowed him to finally create a plan of action. Grandison escapes slavery with his entire family, showing that not only is the "happy slave" stereotype an utter fabrication, but that he is far more intelligent than his supposed master.