Still another possibility is that "Michael Corleone says hello" was part of an earlier script following a different plot. GF and II went through many script iterations that were written and abandoned, though parts of some were actually filmed.

Some members here from time to time have posted parts of earlier scripts that have markedly different outcomes in key scenes. One such earlier script has Roth and the Rosatos arranging Frankie's survival, which would make Aiello's line fit. FFC abandoned that idea because, I'll bet, he saw that Roth would not credibly have risked everything on a hair-raising, incredibly difficult, dangerous, split-second-timed scheme to save Frankie so he could testify against Michael months down the road, when in fact Michael would be long dead--in Havana, at Roth's hand. Perhaps FFC had already filmed the bar scene with Aiello's line and simply decided to save the scene and not sweat the line that no longer fit.