The German midfielder joined Tottenham a year ago, but he has failed to hold down a regular starting berth at White Hart Lane.
Former club Schalke are keen on bringing him back to Gelsenkirchen, but until a player leaves there will be no room on the wage bill for the former Germany Under-21 captain.
"We can't really imagine such a transfer financially," Schalke coach Jens Keller told Germany's Bild newspaper.
"Talks have not yet reached an advanced stage."
Holtby is Keller's preferred choice to replace Jermaine Jones, but until the United States international finds another club Schalke have no leverage in the transfer market.
They have yet to receive a single enquiry for the 32-year-old, who chose not to travel to the club's winter training camp with a move expected this winter.
"Lewis has already played as a defensive midfielder under me," added Keller, whose side are
into the last 16 of the Champions League, where they will face Real Madrid.
"He can play there and is tactically very well-schooled."