Spurs have sealed a pre-contract agreement with Holtby, with the 22-year-old all set to make a move to White Hart Lane as a free agent at the end of the season.
Andre Villas-Boas has, however, expressed an interest in pushing through a move during the current window, with Spurs having lost Sandro to a long-term injury.
Holtby would provide useful cover in the heart of midfield, but Schalke are refusing to lower their demands and are prepared to prevent the Germany international from leaving until the summer.
The Bundesliga outfit also claim that they are yet to open discussions regarding a proposed move for Lyon's Michel Bastos.
General manager Horst Heldt said: "Spurs have increased their offer for Holtby, but only slightly. It is still not acceptable. We have to wait.
"There's nothing new to add on Holtby or Michel Bastos. We haven't opened talks with Lyon yet.
"Tottenham have only marginally improved their offer for Holtby, but it remains unacceptable for us. We will have to wait and see what happens.
"Feasibility is what it's all about, not wishes."