The 22-year-old midfielder has signed a pre-contract deal to move to White Hart Lane in the summer, but Sandro's injury led to the club attempting to rush a deal through
The north London club already have an agreement in place to take the 22-year-old to White Hart Lane upon the expiry of his contract in the summer, but have moved to purchase Holtby in January after Sandro suffered a season-ending knee injury.
However, Spurs will have to up their offer if they are to succeed in their pursuit of the young midfielder according to Heldt.
"Spurs have increased their offer for Holtby, but only slightly. But it is still not acceptable. We have to wait," he told reporters.
"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 to see what happens.
"Feasibility is what it's all about, not wishes."
Schalke coach Jens Keller had stated he would like to keep the young star until the end of the season, but 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," he 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.