The move frees up one of the club's three berths for non-European Union players as allowed under Spanish football federation rules. The other two are taken by Uruguay defender Diego Godin and Brazilian striker Diego Costa.
The 22-year-old Aguero, nicknamed 'Kun', was signed from Independiente for 23 million euros in 2006 and is in the process of negotiating an improved contract with the club.
He has developed a flourishing partnership with Uruguay's Diego Forlan up front and has scored five league goals so far this season.
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