The Eredivisie club are determined not to sell Ejuke, 22, unless a huge offer is made.
An bid from China worth €7m was rejected in May, and interest has only grown since then.
But technical manager Gerry Hamstra said no offer has come close to tempting them into a sale.
Given the strong interest in Ejuke, Heerenveen believe they can get one of their biggest transfer fees in recent years.
They sold Michel Vlap to Anderlecht for €7m last year and expect Ejuke to fetch even more.
Ejuke scored nine goals and registered four assists in the 2019-20 Eredivisie season.