Milan will probably want to buy Zlatan and Boateng. That's gonna cost them around 30m eur only for the transfer fee. Then you have the huge wages Zlatan gets. To fund all that they will probably need to get rid of some others with big wages and get money by selling players too.
Selling the always injured Pato, letting Dinho and his big wages go will cover the amount they need to spend for Zlatan and Boateng. Cassano (for free?) will actually replace Pato and Dinho.
I have no idea if that's their plan, but if it is, it's not a bad one.