Billy Joel has parted ways with his enormous Miami mansion, TMZ has learned -- and the guy who bought it from him is a mega-mega-rich fashion mogul.
Sources close to the deal tell us, the sale went through Friday for just under $14 million. The 7-bedroom, 8.5-bath waterfront super house has been on the market since May 2012, when Billy listed the home for $14.75 million.
And the mystery buyer -- a guy named Diego Della Valle ... owner of Tod's shoes and majority shareholder in Saks Fifth Avenue. He's worth a reported $1.55 BILLION ... nearly ten times what Billy's reportedly worth ($160 million).
Billy bought the place for $13.5 million in 2006 -- but after he pays the agents' commissions, we're told the singer will actually have lost money on the deal.
Oh yeah, the house (8,881 sq. ft) is also located on Miami's exclusive La Gorce Island ... where a bunch of celebs have owned homes, including Cher and Lil Wayne.