I can fully understand why Sancho left for Dortmund, he had no chance at all of making the city first team inside the next 2 years and if we're been realistic he's a lot more physically developed than Gomes and a lot nearer to the first team football. I still think he'll stay and put it down to him wanting a better deal on the table. The longer he drags this out the more likely it is he gets a better deal as United really won't want a repeat of what happened with Pogba.
People don't leave United to join Dortmund for the money. If he's indeed thinking about such a move it's because he is concerned about his personal development. So what he would be looking for is probably assurances from the coach - good luck with that strategy against Mourinho.