Maybe I am wrong, but I don't think that is the scenario here. He has signed a deferred wages contract where-in he is entitled to only receiving small amounts in his first 2 years. And the compensation for the first 2 years, kicks in only in the 3rd year. So if Barca has paid him the low wages for the 2 years, they have honored the contract. They don't owe him anything right now.
But if he refuses to go, then they are forced to pay the compensated wages in the 3rd year, which they are trying to work around, by selling him to us. But FDJ wants to be compensated adequately, for doing Barca the favor of going away.