Kroos, 24, is a long term target of Manchester United and today it was confirmed that the two clubs had agreed a £20million deal.
Hopfner was asked about Kroos' future with Bayern Munich and he told Kicker : "It was the same with Michael Ballack, and eventually we retracted our offer (to extend his contract). Kroos has a contract until 2015.
"There will definitely be further talks. At some point, the player has to decide what he wants. I'll be taking a leaf out of Uli's book certainly: certain financial boundaries will not be crossed."
When asked to talk about how Kroos' offer won't increase Hopfner said: "That is how things stand. No question," but when asked whether a big offer could change their stance Hopfner replied: "I don't know. Things change very quickly."
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