Spurs have been pursuing West Brom striker Saido Berahino but with their bids failing to persuade Albion to allow their man to leave, Son has emerged as another target.
The 23-year-old South Korean – who scored 17 goals last season – was left out of his side’s Champions League qualifier with Lazio on Wednesday due to illness, but Voller says a move could be in the offing.
“There is a request and there are talks. But nothing is done. Son would not have played today anyway, he was sick at the weekend already. At this moment he is our player,” he is quoted as telling Germany TV channel Sky.
“I saw many medicals in this business where there was no deal afterwards.
“We will try to reach an agreement, but only under our conditions.”