I'm not fully aware of Dortmund's financial situation, in truth, but if they're as well off as you suggest, they can reinvest the £100+ million back into the squad entirely. You'd think they'd be desperate for this deal all things considered.
Sancho would be tying himself to Dortmund for God knows how long should he accept a new deal, though. Wouldn't he be better served to wait out the year at his current club, allowing for his market valuation to drop significantly in the process, then gaining a bumper signing-on clause the following summer when United come calling again?
That's what I would do if I was Sancho anyway.