BILD says the centre-back, under contract until 2027, is attracting interest from Real Madrid.
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Real see Schlotterbeck as a potential defensive leader should they make changes to their back line, with doubts persisting over the future of Toni Rudiger.
Bayern Munich are also open to the bidding if space becomes available in their defence.
Dortmund, who see him as a future captain, want to avoid a free transfer and have set his price at around €50m, pending Schlotterbeck's decision.