I always interpreted it as Roth and Ola promised to praise Fredo to Michael after closing the “deal”. Despite what some say Fredo wasnt looking to harm or betray Mike he just wanted to be looked at as reliable in the family business. Obviously we know he wasn’t smart but I don’t think it was for greed. I the “something in it for me” was for him to improve his reputation amongst the Corleone Family as a true underboss