I don't think he is as bad as people here make him out to be. He was full of potential that, for some reason, never developed. Maybe he moved to early to Bayern or whatever.
From a technical point of view, he is skilled and could be of help. What I don't like about him tho is the physical part of his game. He is, to a certain extent, pretty slow, not really strong and I noticed from watching a lot of BVB games both recently and back in the days, that his stamina is not great either. He also has a tendency to miss quite a lot of games due to injuries.
With that said, the area where he excels is in front of the opposition's box and inside of it. Good vision, can pass the ball and he positions himself well. The numbers have not been great the past couple of seasons.
Dortmund sold him to Bayern for 37M and got him back for 22M. In the case of us wanting him as a rotation player with a chance of him proving himself in order to gain minutes and at a good price, I wouldn't be against it.
A fee around 15M would put him in the same area of players like Shaqiri. Low risk, potentially decent reward. But once again, this is only if we bring him in as a rotation player and not as the AM deal that is supposed to replace Fekir.
Anything from 12M to 18M (give or take 2 million) and I'm in. Also, the manager he had the most success under is undoubtedly Klopp. He wants to rejoin Klopp, Dortmund deems him surplus or need the funds? Bring the lad in, but only if we pay a very very reasonable price for him.