Nilssen has received a note from Shepard demanding that he come to the Police Camp. It was marked as urgent.
When he gets there, Shepard accuses him of having broken their agreement to keep the arrangement with the jail money private.
Nilssen isn't sure what to say to get him out of this mess, so he ends up breaking another confidence by telling Shepard that Lydia Wells was Lauderback's mistress. As a result, he claims, Lauderback was then blackmailed, and Crosbie Wells took home the ransom (which is how it ended up in his cabin). Lauderback asks if Staines was the blackmailer, and Nilssen says "yes, maybe."
He also tells Shepard about the deed of gift that Devlin found (and where it's located).
As far as how Lauderback found out about Nilssen and Shepard's arrangement: Nilssen leads Shepard to believe that he told Devlin, and Devlin told Lauderback.
Shepard then goes and gets the deed from Devlin. The two men discuss the deed and what it means. Devlin suggests that Wells was holding that fortune for Emery Staines. Devlin explains his motivations for keeping the deed private.
Devlin asks Shepard what he makes of the deed, and Shepard concludes it was just a joke or a trick.
Devlin leaves, and Shepard burns his contract with Nilssen for the four hundred pound investment. He then writes a letter to the paper saying that Nilssen had donated the money—and calling Lauderback out for his affair with Lydia Wells. He sends two copies—one to Ben, and the other to Nilssen.