The vehemence and emotionalism of Fredo's outburst in the boathouse scene tells me that his motivation in cooperating with Ola was primarily revenge on Michael for having been passed over and being shunted to Mickey Mouse nightclubs, etc.; as well as an obsession with "respect." Ola/Roth would have seen his lust for revenge and respect, and would have figured, correctly, that Fredo'd swallow any tale, any promise (no matter how specious) to get his chance at both.
I agree with this.
I always assumed that Fredo's "something" was a cut of the deal, once it was made on terms favorable to Roth. I mean a cash payout, which would be easily hidden from Michael.
But the real motivation was to get revenge and respect (I think more of the latter). Fredo could be a big
macher , an essential part of a major transaction. He could then convince himself that he could have been a good Don and was unjustly passed over.
