The Sweden fullback had spent part of the past season on-loan with Benfica which included a permanent option of €10m.
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Now Roma have confirmed the sale of Dahl to Benfica for €9m.
Benfica, meanwhile, have stated Dahl has now signed a deal with them to 2029.
Dahl, 22, only joined Roma last year from Djurgarden, before leaving for Benfica in January.