Well Geary went out of his way to insult Mike personally. He wasn't worried about antagonizing Michael. He underestimated Michael and the Corleone power. So perhaps Michael took it personally and replied with equal condescension. As Michael had likely already decided how he would handle Geary if there were any problems this could have been a source of amusement to Michael.
Nicely said, Lilo. I agree.
To Olivant's point, Michael's cocksure answer wasn't consistent with the general Corleone
modus operandi of making your enemies underestimate you. But then again, as you point out, Geary had already underestimated Michael.
