The film does not make clear the relationship between Michael and his sources of wealth. The novel provides some insight. The Corleone family owned tremendously valuable real estate in midtown NY, whole office buildings, pieces of banks, garment center firms, and parts of brokerage firms as well as four LV casinos. And that was as of the late 40s. In order to avoid IRS prosecution or other federal prosecution, Michael would have disguised his access to those assets and the funds they generated. Also, his buyout of Immobliere would probably have been leveraged.