The North London club already has an agreement in place to take the 22-year-old to White Hart Lane upon the expiration of his contract in the summer, but Spurs have moved to purchase Holtby in January after Brazilian central midfielder Sandro suffered a season-ending knee injury.
Spurs will have to up their offer if they are to succeed in their pursuit of the young midfielder, though, according to Heldt, who also addressed the club's connection to Lyon's Michel Bastos.
"Tottenham have only marginally improved their offer for Holtby, but it remains unacceptable for us. We will have to wait to see what happens. Feasibility is what it's all about, not wishes." Heldt told reporters.
"There's nothing new to add on Holtby or Michel Bastos. We haven't opened talks with Lyon yet."
Schalke coach Jens Keller had stated he would like to keep Holtby until the end of the season, but he suggested a transfer fee could be beneficial to the club considering Holtby is definitely leaving at the end of the season.
"Lewis put in a great performance against Hannover. It would be good if he stayed here. But we have to take the financial side into account as well," Keller told reporters.
Holtby's move to White Hart Lane will see him leave Germany for the first time after spells with Alemannia Aachen, Bochum and Mainz before moving to Schalke in 2009. The Germany Under-21 captain has played 78 times for the Bundesliga side and scored 13 goals in the process.