The German outfit have been linked with a move for the midfielder, who spent the second half of last season on loan at Fulham.
And, while revealing his admiration for the player, Slomka cooled any hopes that the club could pull off a deal.
"Holtby is a class player," he is quoted as saying by Bild. "That is not up for discussion.
"But I do not know if it is economically feasible."
"Holtby would be a great addition," he remarked. "It would be great if that would work.
"Lewis is flexible, he has great qualities."
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