The striker spent the second-half of last season on-loan at RBL, where he impressed management.
RBL are now pushing to sign Lookman outright, making offers of €14m and €16m, reports BILD. Everton are prepared to sell for €20m.
RBL chief Ralf Rangnick said: "Ademola Lookman is currently a player of Everton. He knows from his time with us and also through the relationships he has built up with us, our appreciation for his quality as a player and as a person.
"We are in constant contact with his club."