Schalke general manager Horst Heldt has not ruled out selling players this month amid talk Lewis Holtby could make his move to Tottenham during the current transfer window rather than in the summer.
Spurs have agreed a deal to sign the 22-year-old midfielder on a free transfer in the summer when his contract at Schalke expires.
But speculation has emerged the Premier League club could move to bring him to White Hart Lane earlier than expected after midfielder Sandro suffered a knee injury which requires surgery.
Heldt, who recently wrapped up the signing of Dynamo Kiev forward Raffael on loan, has fuelled talk that Germany international Holtby is on his way to North London this month by revealing players could leave.
He told Die Welt newspaper: "This transfer window will not close until the end of the month. We have signed Raffael and it could become a fact that we will sell or buy one or more players this month.
"We will have to wait on a certain progress in some affairs."