Bayer Leverkusen expect Borussia Dortmund to trigger Gonzalo Castro's €11 million release clause, sporting executive Rudi Voller has told reporters.
Leverkusen midfielder Castro, 27, is set to leave the club he joined as a 12-year-old in 1999 after nearly 300 Bundesliga appearances.
Voller told reporters that BVB sporting director Michael Zorc had been in touch with him and added: "Normally, there would have been no need to do that, because of the clause, but I can understand that they tried to reduce the price a bit.
"But we won't allow him to leave for less, and so BVB have to activate the clause -- and it appears that they'll do it, just like we had expected."
Castro, who has won five caps for Germany and was also linked to Bayern Munich, is set to become the first signing for Thomas Tuchel, who will replace Jurgen Klopp as Dortmund coach this summer.
BVB have been seeking to reinforce their midfield, with Sebastian Kehl retiring at the end of the season and Ilkay Gundogan having told the club he wants to leave.
However, Dortmund have said they have no plans for a major squad overhaul, and Zorc told kicker magazine this week that Germany centre-back Mats Hummels will be staying at the club.