Nkounkou joined Saint-Etienne in the January transfer window after cutting short a loan spell at Cardiff City to answer the call.
He had a real impact and helped Saint-Etienne avoid relegation from Ligue 2, leading to the club triggering an option to sign him on a permanent basis.
He has been vocal about wanting to move on from Saint-Etienne and an agreement is in place for him to join Eintracht Frankfurt this summer, despite a move initially seeming tough to do.
According to Sport.fr (via Fussball Transfers), Eintracht Frankfurt have agreed to pay a fee of €8.5m to Saint-Etienne for the signature of Nkounkou.
His imminent departure from Saint-Etienne is also set to give a cash boost to his former club Everton.
The Toffees have a 30 per cent sell-on clause in the agreement that saw the defender join the Frenchman on a permanent deal earlier in the window.
Once the transfer goes through, Everton are due a significant sum for their coffers, which could be vital for their own transfer plans this summer.