It's interesting for sure, because from an accounting point of view, there's no other way to have deferred those wages - other than the manner in which they did it. If FDJ was legally entitled to it, it would have been needed to be put through the books those seasons as incurred, even though no "cash" payment was made to FDJ ergo Barca's wage bill would not have been reduced by the deferred payments.
Maybe they should never have agreed to it in the first place, but then the club might have gone under/not been allowed to register enough players to play etc. and this is the kind of shit pressure they put on all their players.
At the very least, his advisors should have put some penalty clause in place to state that if FDJ were to leave Barca/if Barca were to sell him before the contract period ended, he'd have needed to be repaid a value similar to what he deferred etc.