I know Danny Aiello has claimed ownership for the line, but it is very similar to what is said to Fabrizzio just before he is killed in the book. Maybe they were playing with the line during filming, trying to get it in, but it seems very likely to me that the book is the real source.
I remember that line from the book. That, too, would have made no sense. If Michael, like Vito, needed to provide "buffas" between himself and the underlings who committed crimes on his behalf, why would he have the actual killer tell Fab: "Michael Corleone says hello." If the killer had been caught, or squeezed for another crime, he'd have that to offer the police.
Then again, Michael had three witnessess--Neri, Rocco and Tom--when he gave orders to have Frankie and Roth taken care of. Dangerous! Maybe he was slippin'...
