Everton held talks with Besiktas president Fikret Orman on Wednesday as they attempt to finally conclude a £27m deal for striker Cenk Tosun.
Tosun, 26, has been in London awaiting permission to complete his move.
But it has been delayed as Besiktas hold out for a better deal than the £25m Everton thought would be enough to get the transfer over the line.
Tosun has helped Besiktas reach the last 16 of the Champions League, where they meet Bayern Munich.
Everton manager Sam Allardyce has identified the German-born Turkey international as the forward to add goals and threat to his squad following the failure to replace Romelu Lukaku after his £90m move to Manchester United.
Orman flew to London to try and reach an agreement with Everton as a transfer deal they hoped to do swiftly once the January transfer window opened threatened to become drawn out.
There remains a strong belief that a deal will be done before the weekend after amicable talks in London.
Tosun is keen to move to the Premier League and join Everton, who remain optimistic he will become Allardyce's first signing since succeeding Ronald Koeman.