The Serie A club upped its offer to 12 million euros but has been met with resistance by Spurs, and talks are at an impasse with Rafa Benitez's side so far unwilling to budge.
Even so, Rafael Benitez's side is willing to match Sandro's current wages and believes that offering nearer to €15 million would prove successful, although talks are now at an impasse.
Sandro appeared close to the exit door after receiving strong criticism from former Spurs boss Tim Sherwood last season.
The Brazilian then predicted Sherwood's departure at the start of May and insisted he was excited for a new beginning in north London.
Any particular move is complicated further by both clubs' interest in Southampton midfielder Morgan Schneiderlin.
Spurs have shown an interest in the 24-year-old France international, who is keen to quit the club, with Mauricio Pochettino already having swapped St Mary's for White Hart Lane to replace Sherwood.
But Napoli has already discussed personal terms with the player's agent and are willing to up his annual salary from €600,000 to nearly €1 million.
Napoli has also been alerted by Moussa Sissoko's decision to leave Newcastle United this summer.
The Magpies sealed the signature of Siem de Jong on Tuesday evening and Napoli believes they could be content to recoup the fee paid for Sissoko to Toulouse in January 2013.
The Frenchman is keen to join a Champions League club and has privately made it known that he wants to move to a warmer climate than the north of England.