Dortmund made a 20-million euro bid for the 27-year-old earlier this month, but Leverkusen refused to budge over their superior valuation for the central defender, who picked up an injury in his side's 2-1 defeat to Borussia Monchengladbach on Saturday.

He can leave the club for a pre-agreed fee of 12-million euros next summer, but Voller said Leverkusen still would not consider letting him depart for less than what they felt is his current market value.

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"Omer understood immediately that it was not down to us that the deal didn't work out, but Borussia Dortmund have every right to say they are not going to match our valuation," Voller told Sport1 television.

"It happens and nobody is angry about it. You've got to agree with the other club, and we simply couldn't agree with Dortmund.

"We owed it to him that we would listen to any offer, and he had this offer from Dortmund, but it didn't work out."

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