Great negotiators would negotiate on multiple fronts forcing sellers to think of their strategy else they may end up with no money and an Unhappy player. They would start negotiations way in advance so that the manager can start preseason with a settled side. Finally they know the limitations of sound negotiation. No amount of negotiation will lower a fee from 108m to 80m
But that is the point exactly.
To fairly assess the performance of the negotiators, it must be appreciated that the negotiators are like middlemen between multiple parties.
a. The manager states who he wants
b. The owners/board state how much can be spent
c. The selling club states their price
If any of these parties simply refuses to budge, there is nothing the negotiators can do. No strategy, not even negotiating with a 100 players will convince Dortmund to reduce the price for Sancho.
So at this point, the negotiations cannot be blamed. Only the board for not providing funds, the manager for not having a back up player, and the selling club for not being agreeable.