The France international has seen his stock rise rapidly over the last two years, with the 20-year-old now regarded as one of the hottest prospects in world football.
He has drawn admiring glances from heavyweight outfits such as City, Barca and Paris Saint-Germain, but when asked if there is any chance a deal will be done, Dortmund's sporting director told Die Welt: "No way.
"What we've seen [from him] was extraordinary, both in his value for the team and his development.
"You always get the feeling he can do something special, make the difference. He can decide games by himself, even those games he's not really involved in."
Barca are reported to have already lodged a £40 million bid for Dembele, with the Liga giants prepared to go as high as £75 million.
City and PSG would also have no problem financing such a deal, but any of the youngster’s suitors would need Dortmund to open the door for discussions before a move could be made.