I have said this before, and I have come to the conclusion that Fredo knew it waas going to be a hit and he lied to Mike.
In the conversation with Ola Fredo also says that he is in enough trouble as it is. I think the "lie" he refers to is that there would be something in it for him on hs own if he helped him.

What Fredo tells Michael in the boathouse scene is simply ridiculous, and it shows that he was not smaht at all. Fredo's big issue is that he was passed over, that he didn't really do anything in the family business except pick people up at the airport and run mickey mouse night clubs. Since that was the case how in the world would Fredo have been able to persuade Michael to go easy on some undefined "negotiations" in exchange for something "on my own?"
Fredo did not participate in Michael's negotiations with Roth, and that is made very clear when Fredo nervously shows up with the money in Havana and asks Michael what is all of this about.