David said he is leaving at the end of the season...even if he stays, we will have to invest in another striker. We sell Amauri, we have to buy another top class striker, all the while looking for a LB/RB and a Cannavaro replacement, NOT TO MENTION the next Alessandro Del Piero.
David, more than once, said that he's leaving and changed his mind later. With his current form, It's time to change his mind again and end his career in Turin.
I'm sure we'll have a good transfer budget in the summer without selling anyone.
So, the 20 million pounds will be invested directly to bring a replacement for Amauri.
Selling other players, like Tiaga, Zebina, PDC, we'll certainly have enough budget to bring two FBs and a defender.
About Ale's replacement, IMO, we're covered with Giovinco.