Marcos, the issue of why Carmine Rosato said, "Michael Corleone says hello" if he intended to kill Frankie, is one of the two or three most-often-posted issues on these boards:

The simplest explanation is one that has been uncovered by Godfather scholars in an interview with Danny Aiello, who played Carmine Rosato. Aiello admits he ad-libbed the famous line, and Francis Coppola permitted the ad-lib to remain in the film. I believe FFC allowed it because it fit the plot, even though Carmine intended to kill Frankie all along. “Michael Corleone says hello” was intended not for Frankie-—but for Richie, the bartender, whose ginmill was being used to set up Frankie.
It’s obvious that Richie is a “civilian,” not a Made Man, and he’s nervous as hell about his bar being used for a murder (“Carmine, NO, not HERE!” he screams after the cop enters and Rosato draws his gun). Carmine knows that Richie might be squeezed by the cops investigating Frankie’s murder. Richie would be too fearful of Carmine to identify him as the killer. Still, as a civilian, Richie is not bound by the code of omerta. So Carmine hands Richie something he can give the cops so that Richie can get off the hook: “The murderers said, ‘Michael Corleone says hello.’ ” That line would set the police after Michael, and would be picked up by the press-- another nail into the coffin of Michael Corleone’s “legitimacy.”