They signed him on a free (I assume some compensation as he was under 23?) so the mark-up is relatively high. You can almost be certain the deal will be riddled with clauses; either high sell-ons or low buy-backs.
Also, Barcelona, dodgy deals? This summer gone they did this (Don't want to post the tweet as it will embed):
"Barcelona will pay Valencia an amount of money before June 30 for goalkeeper Neto to balance Valencia's sales budget. One day later, on July 1, Valencia will pay Barcelona the exact same amount back for Cillessen. [tribuna deportiva]"