The 21-year-old forward has been linked with a move to Stamford Bridge this summer, with Leverkusen keen to hold out for a £90million transfer.
Frank Lampard's Blues, however, are only prepared to pay £70million for the German international at the moment and Bayer chief Fernando Carro admits that a potential deal may not work out.
The CEO told Kolner Stadt-Anzeiger: "It was always clear that we will have talks once the Europa League season concluded.
"Kai has voiced his desire to make the next step. At the moment, I can't say whether this will work out in the near future or not."