Because we have clause(s) on him. Among them 50% of next transfer fee UNLESS he's sold back to us. He has release clause almost 200m, but for us it's somewhere around 30m.
We sold him for "peanuts" back then if you compare what we could've gotten in competive market. But it didn't mean we were robbed as we managed to squeeze those clauses on the contract mainly I think because Barca believed in him becoming even better player for them than he was for us.
Those clauses were released after the deal and were pretty widely reported.
I doubt it as if Barca sell him for 30 Million then Barca will be making a loss of 15 Million on a player.
I believe it's some % of the profit and not on the transfer fee.
Any links for the clauses?