In "The Goophered Grapevine," how did the author, Charles W. Chestnut, incorporate folklore into the story to subvert white power?

In "The Goophered Grapevine," a "goophered" is a Southern vernacular term meaning "bewitched." The author presents the character of Julius, an elderly African American man who has outlived slavery, as a trickster figure.
The trickster, who appears in the folk tales of many African cultures, came to occupy an important place in African American oral tradition as well. Julius aims to undermine the confidence of the Northern buyer of the former McAdoo vineyard by telling him stories about the supernatural activities on the North Carolina estate.
Julius frames (and thereby enhances the credibility of) his stories by attributing them to McAdoo's own actions: it was a powerful white man, he claims, who requested that his grapes be bewitched. Although the power to conjure this spell onto the fruit was in the hands of an African American woman, according to Julius, Peggy was only doing McAdoo's bidding.
The yarn that Julius spins has direct parallels with the new owner's situation. In the story, Henry, a new arrival who did not know about the spell ate the grapes but was forgiven because of his ignorance. Because this African American man did commit an infraction, he was punished—but not harshly; he was bewitched and actually became part-grape. The white Northerner who tried to improve the vines' output, however, ignored the spell and killed not only the vines but also Henry.
Julius uses this elaborate tale to try to dissuade the new owner from making any changes, which would probably result in his being dismissed. Julius's narrative supposedly puts the power to effect change into the white owner's hands but clearly conveys the superiority of the supernatural power invested in the enslaved black conjure woman. Furthermore, it also puts forth that outsiders will have a negative effect, while those already familiar with the vineyard can make it produce.
The elderly man's plan does not accomplish his goal, however. The Northerner is not as credulous as Julius thinks. He sees through the ploy, realizing that Julius is acting out of self-preservation to discourage his purchase.