TB, I think it's plausible to think that Roth just assumed that Fredo brought the money ("in the bag in your room"); it's also plausible to think that Fredo had told Roth about it. In fact, at that point in the story, is there any reason to believe that Fredo had severed his relationship with Roth? While at the outdoors bar with Michael Fredo laments the way things turned out, did he still believe that his best shot at respect was with Roth? If so, then it is Fredo who informed Roth of the money.