except we do know, courtesy of football leaks
Barca paid €70m upfront for Dembele and will have to pay another €35m at the end of the season. They also have to pay €5m for Dembele's 25th, 50th, 75th, 100th match, or the whole sum should he move before achieving those numbers.
There are also €5m payments for qualifying for CL and winning it, that can be triggered twice each - though I don't know if they also apply if he's sold prematurely, like the player performance related add-ons.
And in your example you're proposing that Barca sell a player that has already cost them at least €130m (possibly €145m) to sign for €80m.
That's what I meant earlier. You don't give up on such an investment, such a talent at such an age if you have to take a massive loss and if actually do then you send out a warning signal to every potential buyer that the kid first went on strike at Dortmund and then must have behaved badly enough for Barca to accept huge losses to ship him out. Even in football I doubt that clubs would just ignore that.