You are confusing different things
-For the FFP, it was important that Dembele renews that January. The threat did not work there, as it could work only if he renewed
- As for losing Dembele for free AND paying his exorbitant wages, the threat was still effective, and had it turned into a punishment, Dembele's value would have dropped, and that could have caused drop of interest in him from other clubs (and thus lower or no offers)
PS: In general, any club should resolve renewals before final year of contract.
If there is no agreement, and the player refuses to go the final summer before the last year, then there is no more space for negotiations.
1) Numpty clubs will say 'let's play them they may help us'
, giving stupidly away their only weapon, and letting the player have power, with his stock high, and offers from outside multiplying so he not only leaves on a free but with signing bonuses and better wages
2) Clinical and serious clubs send them to the stands, reducing their value, which doesn't concern them at this point, but concerns very much the player themselves and their future clubs.
Offers are getting reduced, and the player is risking a lot in insisting, as they are jeopardizing their career by accepting a year of inertia
Elementary stuff