Ok but you can totally do what he's doing. There's some price that would get Microsoft to sell Wunderlist, and if he's willing to pay that much then he will get Wunderlist back. If that's "backsies" then you can absolutely backsies as much as you want.
That's why this feels like a publicity stunt to shame Microsoft.
If you've made millions of dollars from selling something - I don't feel like you have the right to shame the buyer especially if you're only offering 25% back. He should have kept the offer he made private.
Your can't have your cake and eat it too. You can't sell your company and keep it too. Basic.