However, club president Giovanni Luigi did not specify which club had made the offer for the 20-year-old, and said in any case that the amount offered did not meet their own valuation.
"We received an email today, but it's a long way off being done," Luigi told Radio Gaucha.
"The proposal wasn't as good as expected. We think Oscar is worth more than €20 million (around £15.7m).
"It wasn't an offer - it was more like an enquiry. We are only saying that the numbers have to change."
Oscar – full name Oscar dos Santos Emboaba Junior – will be part of the Brazil squad competing at the London Olympics this summer.
Brazil face England in a pre-Games friendly in Middlesbrough on July 20.