After listening to “The Beast” in its entirety, your hunch that Payton and Grams are mistaken would seem to be by substantiated by the similarities between Agate’s adaptation and Wharton’s 1908 story “The Choice” (first published in _The Century Magazine_ here: http://www.unz.com/print/Century-1908nov-00032/) and republished in her 1916 collection _Xingu and Other Stories_ (available online here: https://bit.ly/2tfoyKv). While Act I of the adaptation (including the accident, the dead man in the road, the dead man’s son seeking revenge, and the police investigation) is almost entirely Agate’s invention, the first part of Act II and much of Act III closely follow the plot of Wharton’s “The Choice” and directly quote her dialogue on numerous occasions.
There is a faint possibility—since the central character in “The Beast” claims, at one point, that he hit a dog (not a man) in the road and Wharton’s “The Blond Beast” includes an important episode involving an injured dog in the road—that Agee’s first act may have been partially inspired by Wharton’s other story. The similarities between the two titles could, however, be entirely coincidental, since it’s made clear in the course of “The Beast” that its title is (at least in part) an allusion to Hamlet’s soliloquy (quoted here from the 1604 Second Quarto, Folger Shelfmark: STC 22276) in which he exclaims, “What is a man / If his chiefe good and market of his time / Be but to sleepe and feede, a beast, no more:” (http://www.quartos.org/main.php).
Best of luck with your book,