And with Chelsea not in contact as yet, De Laurentiis said Real are the only club to have made an acceptable offer.
Cavani, who scored 38 goals last season, is one of the hottest properties in the world game and has been consistently linked with a big-money move.
But it appears City's interest in the 26-year-old, who scored three goals against them in two Champions League games in 2011, could be cooling.
De Laurentiis said: "Manchester City have not the money to pay for Cavani. The Abu Dhabi (owners) have said they will never spend 63million euros for Cavani.
"They say, 'We can give other things', but they are not in the interest of our club."
The name of City striker Edin Dzeko was mentioned later in the press conference.
De Laurentiis added: "Chelsea? Certain kind of agents may tell you Chelsea will call you, but maybe he has spoken with somebody who has spoken with somebody, who has spoken with somebody who says 'maybe'.
"My company never received a call. It is very difficult to pay the 63million (euros) because Financial Fair Play is conditioning everybody.
"At this time, Real Madrid are the only club who have made a firm offer for Cavani.
"It is human, if Real Madrid call, to answer. He knows he has a clause and must decide."
De Laurentiis was speaking at a press conference to unveil new coach Rafael Benitez, who professed himself delighted to have joined a Champions League side who are "still growing".
Benitez was announced last month as the successor to Walter Mazzarri in the Naples hot-seat and said yesterday: "I am very happy, very happy.
"A lot of journalists are here from different countries, that shows the club is growing. We are in the Champions League and still growing."
Napoli secured their place in the elite European competition by finishing second in Serie A last season, their best result since they last won the Scudetto in 1990.
De Laurentiis is aiming to push on further from that mark, and said: "Rafa Benitez is an international name.
"We have made very big progress in the last four years and I hope in the next five years to jump again and again and again.
"Benitez will help in that process of going up, with results which will put us in the condition to play - I hope for ever - in the Champions League."