Club sporting director Luis Cesar Souto claims the offer was far below their valuation of the Brazil international who they maintain is not for sale
The 23-year-old has repeatedly been linked with a move to Europe, with Olympique de Marseille and Tottenham among those rumoured to be interested, with the latter long-time admirers.
However, Souto - who recently slapped a €25m (£21.5m) price-tag on the Brazil international - has confirmed Napoli have since entered the race to sign the striker, but insists they remain determined to keep hold of their star man.
He told reporters: "We had a verbal offer presented by Napoli of €18 million. We do not accept that this is Leandro's true value. What Napoli have offered is very far from what we expected the bid to be.
"We believe he will be part of the Brazil national team for the 2014 World Cup. We have not yet given them a fixed price because it is not in our interest to negotiate for him.
"We have just sold Fred and Rodrigo Moledo. We don't have to sell Leandro and we won't be taking an offer for him."