I think it's possible--likely--that Ola/Roth didn't offer Fredo anything specific--they didn't have to:

They figured Fredo was deeply resentful of being "passed over" for the Donship. So, when Johnny "bumped into Fredo in Beverly Hills," he probably bought him a few drinks, then commiserated with Fredo ("a shame about you being passed over...you, the older brother...")--just what Fredo bellowed about in the boathouse with Michael.

Then, in the scenario I'm envisioning, Ola told Fredo what Fredo told Michael: That Roth was negotiating a big deal with Michael, that Michael was being tough on the negotiations, and that if they got some information, it'd be "helpful to the family--and that there'd be something in it for Fredo." Fredo would have jumped at it, but Ola wouldn't have told Fredo what he needed to do--and what he'd get--right then. He'd tell him to wait for further word.

Once they firmed up the Tahoe shooting plan, Ola called Fredo and told him to open the drapes. Fredo was stupid, but he had to know immediately that they weren't interested in ogling Kay getting undressed--they were going to ither kidnap or assassinate his brother. That, I believe, was just what he wanted: to step in and become the Don, as he thought he deserved. He wouldn't have been satisfied with owning or managing a casino in Cuba or anything less. And, I think, stupid though he was, he'd figure out that he couldn't reap a big reward from Roth if Michael survived--it had to be all or nothing for Fredo. The Donship was the prize; and once he figured out that Michael would be removed, one way or another , he didn't have to ask Ola again about what was in it for him. That's why, in the boathouse, he said: "I swear to God I didn't know it was gonna be a hit." What he meant was that Ola didn't specifically tell him it was gonna be a hit--Ola didn't have to.

To Lou's point about a "shiny dime and a new hat": Yes, a constant theme in the movies and in the novel is that greed and thirst for revenge motivate and undo stupid people. How could Paulie think he wouldn't be the immediate suspect in Vito's shooting? How could Carlo think he could get away with beating Connie a second time to lay the ground for Sonny's assassination? And, how could Fredo think he qualified to be Don, and would be able to run the family after conspiring in his brother's murder?