In chapter 14 of The Ruby in the Smoke, what bargain are Mrs. Holland and Mr. Selby agreeing on?

Mr. Selby arrives at Mrs. Holland's home frightened. He is convinced that he is being followed and that his life is in danger, so he has sought out Mrs. Holland for help. He believes that, if he reveals the whole story of his "secrets" to Mrs. Holland, she will protect him. Mrs. Holland remarks that she already knows all of Mr. Selby's secrets; however, he informs her that she doesn't know the entire story.
As it turns out, Mrs. Holland is interested in his story, and they agree to fifty percent of the profits each in exchange for his information. But Mrs. Holland would like for Mr. Selby to ascertain the whereabouts of Sally Lockhart in exchange for her protection of him. Though this makes Mr. Selby uneasy, he reluctantly agrees to this "bargain" with Mrs. Holland. Afterwards, it is said that Mr. Selby believes he has made a mistake, but the deal is already done.