If we are offering them, with easily reachable add-ons, something at +/- 100m euros they'd be mad not to accept (this isn't just my Man Utd glasses on).
When looking at the economics of this - the likes of Barcelona, Real, Bayern, the big PL teams, practically all of them are already at the point of having to sell before they buy in order to protect from the continuing Covid crash. As others have said it's clear there aren't going to be fans in stadia for at least 6 months and so, come summer 2021 this situation is only going to be worse. Sancho will at that point only have 2yrs left of his contract on a lot of money and no club will be walking up to them with a nine-figure offer.
TBH I could see them rejecting this, Sancho making a transfer request, then United putting in a 2nd bid in the middle of this one and the 120m Dortmund want which will get it over the line.