Sadran is quoted as telling L'Equipe that other clubs will have to match the bid from Cardiff to stand any chance of winning the race for his signature.
Midfielder Capoue has been linked with a move to English football for some time and Queens Park Rangers are rumoured to have had an offer for him rejected in January.
Sadran recently admitted that he would be prepared to let the 25-year-old leave for the right price, and reports in France now claim a transfer fee has been agreed worth €10million plus add-ons.
It is also reported that Capoue is hesitating over his future as he expected a contract with one of Europe's top sides.
"Now the clubs interested in Etienne have to know we will not accept any offer inferior to the one from Cardiff," Sadran said in L'Equipe.
Cardiff have bolstered their ranks this summer with the record signing of striker Andreas Cornelius, but he is so far the only new arrival and manager Malky Mackay has urged fans to remain patient.
The Bluebirds are gearing up for the new Premier League campaign after winning the Championship and kick off the season away to West Ham United.