I think it's a different thing, like buying the rights to his services as opposed to buying a person, presumably he agreed to the terms of a contract with the British club and the German one simply bought that contract... In becoming self-employed I have found it to be best practice to have language in a contract covering changes like this, specifically that either side can rip up the contract in the event of a covered change. Hopefully this player has that language and he'd be free to renegotiate.