The 23-year-old arrived at White Hart Lane from FC Schalke 12 months ago in a £1.25m deal but has failed to establish himself in England.
Holtby's agent Marcus Noack said: "My feeling is that he will leave but it's not in our hands at the moment.
"Tottenham have to agree to let him go first. We hope to find a solution in the next couple of days."
Spurs are thought to be asking for between £8m and £10m for Holtby's services and Noack added on Spurs' valuation: "I think it will be a loan because it is difficult for anyone to buy Lewis."
Former Germany defender Christoph Metzelder earlier claimed via Twitter that Holtby had been seen in the Dusseldorf area, close to where Schalke and Borussia Dortmund are based.