I also think this has to do with Dortmund not being on best terms with United. It's just an impression but all this insisting on an intermediary, the relatively strict public statements etc.. it seems to me that they're annoyed by United's way of approaching such transfers. Maybe the Mkhitaryan story has also left a lasting impression on them. Then again, I also think the British media are exaggerating a bit. Many incidents (for instance the "you love to see it" tweet) are perceived as affronts towards United but I don't think they were meant this way. Besides that, Zorc was questioned by journalists and the answers were taken out of context. Typical stuff, really.
Another United supporter has demonstrated quite profoundly that Dortmund indeed does rely on player sales as a revenue stream. However, I think that's just a transitional thing and not intended to be the norm. For Dortmund it's important to hold onto their stars for as long as possible. If they manage to keep a team like the current one together for a few years, it'll bring them closer to the ultimate goal of becoming an elite club. It's all cyclical for them and they are probably one or two of those "talent cycles" away from being able to keep the best players for a few years of their respective peaks. Obviously they can't force players to stay because otherwise it will harm they reputation as a talent hub. I believe that's the only reason they were willing to negotiate Sancho's exit in the first place. To me it seems as if they now happily walk away from negotiations because they kept their promise to him.
For what it's worth, Dortmund is getting closer and closer to their goal season by season. Nowadays they already have the
12th highest wage bill and the
12th highest revenue among clubs in Europe. Give them a few years and they're on par with Arsenal, the club that poached Aubameyang from them not too long ago. So it's not completely unreasonable that they could keep this team together for a sustained period of time. Hence the situation surrounding Sancho's transfer has become so delicate. They're "seeing light at the end of the tunnel".