Spurs have already agreed to sign the 22-year-old on a pre-contract agreement which will see him arrive at White Hart Lane at the end of the current season.
However, a recent knee injury sustained by Brazilian international Sandro has resulted in Tottenham wanting to bring forward Holtby's arrival.
Having reviewed their bid , Schalke has described the amount offered as "not acceptable".
"There is a request from Tottenham. The offer is not acceptable," said general manager Horst Heldt. "But the transfer window is still open. Lewis has not become cheaper today."
Holtby found the net as Schalke defeated Hannover 5-4 last night.