"Let he who is without nitpicks among you cast the first stone."

Both SC and DC are right. No sequel was in the works when GF was finished, so the sale of the mall and its property could simply have been a fait accomplias Michael moved the family West. But the "sale" of the property could have been a tax shelter and money-laundering move: Michael could have "sold" it to his New York underboss, Clemenza, for a fraction of its real worth, and taken a big tax credit. Clemenza could then list the deflated price of the mall as his main asset, and escape his own big tax bill. He could also have set up a dummy corporation to buy the mall, then "leased" it from his own company to launder money.