The 22-year-old, who has a year remaining on his current deal, has been strongly linked with a move to Everton at the end of the season.
His agent Thomas Strunz told BILD: "Fabian is doing fine just here. He has a contract with the club beyond the end of the season.
"I am able to confirm that we have received several enquiries from clubs from in and outside Germany regarding Fabian's availability, but I'm not going to reveal any names."
Schalke 04 are also understood to be in the hunt.