Saint-Etienne took up the option to sign Nkounkou on a permanent deal worth €2m earlier in the transfer window.
He was outstanding last season as he helped Saint-Etienne avoid relegation from Ligue 2, but is wanted in Germany this summer.
Saint-Etienne rejected a €4m offer from Eintracht Frankfurt recently as Everton’s sell-on clause would have meant that they would have bagged little money from the sale.
According to French sports daily L’Equipe (print edition), Everton have a buy-back option worth €2.5m and have a significant sell-on clause as well.
The Toffees would be due 30 per cent of any transfer fee if Nkounkou is sold by Saint-Etienne.
Saint-Etienne would have bagged only around €1m if they accepted the offer from Eintracht Frankfurt for the defender.
The French club have received another offer worth €9m from Eintracht Frankfurt, but they have postponed making a decision on the left-back’s departure.
