Answer
In 2009, Borussia Dortmund acquired Mats Hummels from Bayern Munich for a transfer fee of €4 million. Hummels, a German center-back, had previously played for Bayern's reserve team and made a few appearances for the senior squad. His move to Dortmund marked a significant step in his career, as he became a key player for the club and helped them achieve several domestic and international successes.