The Swans had been negotiating a loan deal for a permanent transfer of the Ghana international.
Fosu played 49 games for Brentford last season as they gain promotion to the Premier League.
He is yet to feature in this campaign, with manager Thomas Frank indicating earlier this month that he would be prepared to let the 25-year-old leave.
Fosu joined Brentford from Oxford United in January 2020.
He helped Brentford reach two successive Championship play-off finals, losing to Fulham at the end of the 2019-20 season before beating Swansea in the 2021 final. Read Full Story
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