There must have been the whole deal was set up when Dortmund was able to buy for the bargain price of £20 million, the leverage that low price had was the agreement of another release clause drastically below market value.Even if there were agent fees etc that took the total fee to £100m (there probably aren't), he's still likely to become quite comfortably the best £100m+ transfer of all time. So I don't see the problem.
This is to achieve a large signing on bonus, plus agency fee. It was Mino who was agent at the time of the transfer to Dortmund.