Well, right now I don't think he would even entertain a move to Barcelona. If he's leaving the situation he has in Dortmund, it's to go to a better one without losing his starting job. He wouldn't be a starter in Barcelona, they want him to improve their depth. Eventually though, they're going to make room for him if he keeps progressing the way he has, and at that point it will be nearly impossible to stop the move in my opinion.
This is why I would go for him now, not a year from now. Lock him down now while he's in an evolving situation where his best friend at the club Auba is sold, the coach he enjoyed working with was fired for standing up to the club making them play after the bus incident, there may not be a better time in his career to get him for United.
I know people's position on this board is that he's not for sale, but Dortmund did quote a price to ward off Barcelona, a club with zero intentions to sell wouldn't even quote a fee.