If this supposed correspondence involving Dortmund, Sancho and Utd has been going for at least a year, then it’s going to be pretty extreme from all sides to just stop now, seemingly over payment instalments.
I’m willing to bet the overall fee is agreed, the personal terms are agreed... it’s just a case of how much money is going to be exchanged this year and the years after that. Utd want to keep it as low as possible so they can have more money to spend now on other players. Dortmund want more so they can have more money to spend now on other players.
Sancho will likely be a Utd player at the end of it all regardless and will be in the Utd 20/21 squad, but the media will milk it for all its worth.