Chelsea are in talks to sign the 28-year-old goalkeeper from Rennes and are claimed to have offered €25m to the French club.
There were suggestions earlier this week that the two clubs are close to agreeing on a move, but Rennes coach Julien Stephane said on Thursday that a deal has not been agreed yet.
He did confirm that the two clubs have continued to hold talks but for the moment no deal has been agreed yet.
According to French sports daily L’Equipe, Rennes are holding out for a fee of €30m before agreeing to let Mendy move to Chelsea.
The French club are resigned to the fact that they will lose the goalkeeper to Chelsea but want the best possible deal.
The talks between the two clubs started at around €20m but Rennes are prepared to make sure that they get around €30m from his departure.
Mendy has told Rennes that he does not want to miss out on a move to Chelsea and has already agreed on a contract with the Blues.
Chelsea manager Frank Lampard wants Mendy at his club as competition for Kepa Arrizabalaga.