Here's what I think is happening: United are willing to pay €60 million up front with €30 million in installments and €30 million worth of Add-ons.
Dortmund don't want the Add-ons, they want a guaranteed €60 million installments guaranteed. It's like Scouse said earlier, there is a possibility of a 2nd wave of COVID in the middle of next season which would lead to another shut down which in turn would **** teams over financially. Fans underestimate how much that uncertainty affects transfer dealings.
Fair to say Dortmund would be the main beneficiaries of these uncertain times. They are stable financially so they wouldn't need to sell. If they sell, it's going to be on their own terms. Unless Sancho decides go AWOL (don't see this happening), I don't see a deal being struck. Better to move on now than clutch onto straws