I don't think August is off base. What txts or email records support that Miller knows anything vis a vis the the 30k? He had a conversation with his wife. That's the sum total of it.
Miller's under no obligation to report that. The only evidence he has, and the only reason he knows, is his wife tells him in conversation. If he wasn't involved, and never admits to it, then what possible evidence would the FBI uncover that even relates to him? He's completely unconnected. It's superfluous information that, at absolute best, turns out to be innocuous, but more likely, could be harmful.
It was a weak move imo.
Now, if the FBI had Glen Miller as a high value target, and somehow managed to bugged Glen Miller's house beforehand thereby having that particular conversation on record, then I take all that back.