Hold your ground Ed. 90mm pounds (99mm Euro's,) is more than enough.
Put yourself in Dortmund's shoes. Jadon does not want to be there. Dortmund takes the risk that Jadon could cause problems in the locker room, or could just not try very hard while out on the pitch-- thus leading to him being benched by the manager. Now all of a sudden, Jadon's on the bench and Dortmund can't move him until January when his value will drop to about 50-60mm pounds. Is Dortmund really willing to take a 30-40mm pound risk by keeping Jadon past this summer? Dortmund gets no benefit from Jadon if he's not trying and is on the bench. This just creates unnecessary drama for the club that the manager will not want.
Is Dortmund a club that out of spite is willing to lose a lot of money by keeping Jadon for 1 more year, or even 2 more years and let him walk on a free? IDK. When you factor in Covid, 90mm pounds is more than enough. I'd only offer 80mm pounds, with maybe another 10mm in addons at most.
90mm pounds is more than fair- and if that's the most that Ed wants to pay- then do a take it or leave it offer that expires in 48 hours. Get a yes or no quickly, and if No, then move onto other targets. Don't look so desperate Ed.