The midfielder has agreed to move to White Hart Lane in the summer but Spurs, who are now without Sandro for the rest of the season due to a knee injury, have made an offer for Holtby to bring forward his transfer.
"There is a request from Tottenham. The offer is not acceptable," said Schalke general manager Horst Heldt. "But the transfer window is still open. Lewis has not become cheaper today."
Holtby played a starring role as his side returned to Bundesliga action after the winter break, with a 5-4 win over Hannover on Friday.
Speaking after the game the midfielder was non-committal regarding his future, insisting he was simply concentrating on his football.
"I will give my all for Schalke until the last second. Everything else must be decided by the club."