Well, as MC said, FFC hadn't envisioned GFII at the time that scene was filmed in GF. But, it's a fact that "the house never went to strangers...first Clemenza, then you [Frankie]." I agree that the for-sale sign went up for appearance's sake. If the Corleones had simply given the house or the Mall to Clemenza, he'd have been identified as the "heir" and successor to Vito Corleone, and thus would have been tarred with Vito's public notoriety (think of the headlines when he was shot). This is not to say that Clemenza wasn't a known Mob bigwig, but careful Mob guys never willingly embrace public recognition (think of Barzini ordering the picture of him at Connie's wedding to be destroyed, even though it never would have gotten out of Connie's possession). Then, too, Clemenza would have had to pay inheritance taxes. I'm guessing he bought it through intermediaries who weren't named "Clemenza," and that Frankie got it the same way.