The line ("Michael Corleone says hello") doesn't make any sense (for the reasons stated above). That dialogue wasn't written for the movie, but rather ad-libbed by Danny Aiello. Coppola liked what he heard and without regard to the storyline, he left the line in the movie.

That whole scene was based on a real life incident involving the Gallo Bros and Joe Profaci. A line similar to what Danny Aiello ad-libbed was actually spoken back in 1961 during an attempted hit on Larry Gallo.

Welcome to the boards, mcorleone!