This.
Plus, if the fee was higher, the fees of the buy-backoption would be higher as well. If he turns into a world class player within two years, which is possible considering his talent, we could get him back for under 15M.
Much, much better to sell for 15m and have a 25m buyback option in two years than to sell for close to nothing with a lower buyback option. Now we are not assured of hardly any money. It is far more likely that he will turn into a good player for Everton but not good enough for Barcelona than it is that he will turn into the new Neymar or Hazard. And if he doesnt turn into a player good enough to play for Barca, we wont activate the buyback, leaving us with a paltry 6m euros only.