The midfielder, who has hinted at wanting to play in the Premier League, has a contract with Schalke until the end of the season.
Both Arsenal and Everton are keen on the German international and are expected to try and land him in January.
They could either sign him for a cut-price fee when the window opens or sort out a pre-contract deal, which would lead Holtby to England in the summer.
Schalke, though, are trying to get 22-year-old to sign a new deal, with one final offer on the table.
"Negotiations are in the end phase, it's down to details," general manager Horst Heldt said at a press conference in Germany. "A decision will be made before the year ends.
"I'll only look into it [selling in January] if someone expresses an interest. Our aim remains to agree a new deal."