I've often speculated that FFC left in "Michael Corleone says hello" because it was intended for Richie, the bartender, to hear. He was a civilian, and would be a prime candidate for police grilling after Pentangeli turned up dead, as planned. He obviously would never rat out the Rosato brothers, so they threw him something to tell the cops: "I dunno the guys who done it, but one of 'em said, 'Michael Corleone says hello.'" That would have set the cops on Michael. Too bad for Roth and the Rosatos that the cop showed up and thwarted their plan to kill Pentangeli.