In Fredo's case,I think a safe bet would be a large sum of cash.
After all,if Ola would have promised a piece of Roth's business interests.or something along the lines of bankrolling a Club for Fredo,then surely even Fredo would figure out that Mike would have known something was amiss,if anything like that came to pass.
Even if it wasn't a hit,once Mike saw Fredo in a non-Corleone business 'there would have been a very intense investigation.
So IMHO,the best course of action for Roth would have been to promise a nice fat envelope, perhaps as much as a half million,for Fredo.knowing that Mike would soon be a dead man.
This way,they pay out nothing,and Fredo has no way to collect.
I mean, what's Fredo gonna do,go to Tom and tell him that Roth had Mike killed,and he knows it because he helped set Mike up,but he was duped?