Barcelona announced the signing of Danish defender Andreas Christensen on Monday, hours after the signing of Ivory Coast midfielder Frank Kessie after his departure from AC Milan.
Barcelona said in a statement on its website that Christensen will have a contract until June 2026, and the release clause will amount to 500 million euros ($522.2 million).
The 26-year-old defender helped Chelsea win the Champions League title in the 2020-2021 season, as well as the Club World Cup and European Super Cup in 2021.
Christensen, who made 56 appearances for Denmark, is no longer associated with any team after his contract with Chelsea expired last month.
Casey also signed a four-year contract with the same penalty clause.
The 25-year-old left AC Milan after his contract expired in June after helping the Italian club win their first top-flight title in 11 years last season.
Barcelona finished second in the Spanish League last season, behind rivals Real Madrid, the champion.