The obituary for Bettie Rollins. This obit has several mistakes. Bettie’s husband, William, was not the ferryman at the Rappahannock river. Nor did William Rollins receive any money from the government for Booth’s capture.
Image Source: Richmond Times Dispatch
LincolnConspirators.com 
I’ve read it was James Thornton who was the ferryman and he was never paid by union soldiers for ferrying them across the river to find Booth and later when they returned with Booths body. William Rollins was present but was not the ferryman.
Mari,
You are indeed correct that James Thornton, not William Rollins ferried Booth, Herold, and the three Confederates across the Rappahannock. Since Rollins was present and had interacted with the men, over time the ferry story was misappropriated to him.