Prosecutors have opened a preliminary investigation without citing any specific crime or suspect, the source said.
Parties involved in the sale were not available to comment.
Berlusconi, who served four times as prime minister, sold the club in April 2017 for 740 million euros (£645.6 million) and stepped down from his role as president which he had held from more than 30 years.
In January a report by Italy's finance police pointed to potentially suspicious element in the transaction, sources said.
(Reporting by Manuela D'Alessandro; writing by Francesca Landini; editing by Emilio Parodi)
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