Chelsea have attempted to sign Kai Havertz for €65 million (£58m/$76m) with another €15m (£13m/$18m) included in add-ons - but the offer remains €20m (£18m/$24m) below Leverkusen's €100m (£87m/$118m) asking price.
The Blues had waited until the completion of Leverkusen's Europa League campaign before advancing talks for the 21-year-old who remains their priority transfer target, alongside Leicester City's Ben Chilwell.
The Germany international has already spoken of his intent to leave his boyhood club this summer and he has verbally agreed on the personal terms that the west Londoners are offering.