The Italy international is one of the hottest forwards in Serie A and is believed to be a target for several elite European clubs.
"One month ago, Manchester United made a £10million offer," claimed Quagliarella's agent, Silvio Pagliari. "We believe Fabio is worth between 15 to 20 million euros (£10-13million), hence if the English club's offer still stands, he could probably go abroad."
Quagliarella joined Samp last summer from Ascoli, although his sporting rights are co-owned by both the Genoa outfit and Udinese.
The 23-year-old made 35 league appearances for Sampdoria last season, scoring 13 goals.
His performances earned him a call-up to the national team, and earlier this month he scored his first two goals for the Azzurri during Italy's 2-0 Euro 2008 qualifying win against Lithuania.
With such an impressive start on the international stage behind him, it comes as no surprise that Samp are anxious to keep hold of the in-form hitman.
"More than ever, we want to keep Fabio's services," said Samp's club administrator Giuseppe Marotta. "We will only stop if faced with an extraordinary offer."
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