West Ham have signed defender Manuel Da Costa from Fiorentina, and allowed forward Savio Nsereko to move in the opposite direction.
Fiorentina announced the completion of both deals on Monday evening.
Germany Under-21s international Savio returns to Italy just seven months after joining West Ham from Brescia for a fee of around £9million.
Da Costa, 23, moves to Upton Park on a three-year contract after one unhappy season in Italy.
The defender joined Fiorentina in January 2008 from PSV Eindhoven, having previously been a target of Italian giants Inter Milan and Juventus.
He made just one league appearance for the Viola before being sent on a six-month loan to Sampdoria. His fortunes did not change at Samp, however, where he made just three appearances.
West Ham will retain 50% of Savio's sell-on value in the future after the forward's disappointing spell in the capital came to an end after just 11 appearances.
"This is a good deal for everyone concerned," West Ham chief executive Scott Duxbury told club's official website.
"Although not for the want of trying, Savio has struggled to make an impact for various reasons, while Manuel is also looking for a fresh start.
"We will continue to look at home and across Europe for the brightest prospects as they best fit our vision for this club's future.
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