The German attacker only has just over a year left on his current contract, says the Mirror.
He is reluctant to pen a new one with the Bundesliga side, at least until he sees the strength of their summer recruitment.
His stalling has spiked Spurs' interest.
Mauricio Pochettino knows that he will face strong competition for the 28-year old.
Liverpool have been linked with an interest in the ace for several months.