The La Liga side are banned from registering new players until January and have only offered £30m for the striker so far.
The Blues are understood to want closer to £50m for the Spain international though.
Sky Sports say progress is being made between the clubs and quote a source as saying “it is not sorted yet”.
Costa has been back in his home country of Brazil against Antonio Conte’s wishes.
That is after the Italian told him at the start of the summer that he was surplus to requirements at Chelsea.
The Blues signed Alvaro Morata as Costa’s replacement from Real Madrid but have yet to get rid of the 28-year-old.
Chelsea have told Costa to return to training at Cobham but the forward remains adamant he will not.
The enigmatic striker has won the Premier League twice in his three seasons in England.
Costa gave an interview earlier this summer claiming Conte had treated him like a criminal which was laughed off by the Chelsea manager.
The Blues kicked off their Champions League campaign last night with an emphatic 6-0 victory over Qarabag.
Next up for Chelsea is a tough match against Arsenal in the Premier League on Sunday.
