They probably never told Fredo what exactly he would get, but the prospect of doing something on his own was too tempting for the weak, stupid one.
Exactly. And, when people are out for revenge, they seldom think through logical stuff like consequences or rewards. Fredo's outburst in that scene showed the depth of his resentment of Michael. When Roth and Ola made their proposal, I'm guessing Fredo was focused more at his opportunity to get back at Michael than on the specifics of his reward. Roth and Ola probably knew that, too.
If you buy my reasoning here, you might be inclined to believe, as I do, that Fredo
did know it was gonna be a hit.
