This ignores legal realities about property and transferring wealth. When she buys him for 400mil, she will briefly place the money in escrow, reducing them to 700 and 400mil. Then, when he becomes her property, Taylor also gains his assets, reaching 1.5 billion when the escrow is released.
And to whom then Taylor Swift pays said 400million? They just disappear? Or do you assume Kanye is already owned by someone and the money goes to his actual owner, not Kanye himself?