Turkey forward Cenk Tosun's move to Everton is in doubt after negotiations with Besiktas stalled in the 11th hour amid disagreements over the transfer fee.
The Istanbul club are now reported to be demanding in excess of £27m for the forward, which would be a record transfer fee for a player leaving the Turkish league.
Tosun has turned down interest from Tottenham, Crystal Palace and Newcastle for Everton, after being convinced by the Merseyside club's greater financial clout and ambition.
Toffees director of football Steve Walsh flew to Turkey at the weekend to negotiate his transfer.
"We have done all we can to try and secure the player," Everton manager Sam Allardyce told Sky Sports after the club's 2-0 defeat against Manchester United on New Year's Day.
"Whether that is enough, only time will tell within the next 24 hours."
Allardyce has made the signing of a forward the priority of his January transfer window, with the club struggling to fill the void caused by the sale of Romelu Lukaku to Manchester United in the summer.
"Our ratio of chances created is very limited. That is because we need to strengthen that front-line. I think we all know that," the Toffees manager said. "We are trying to do the hardest job of all by securing a player in that position.
"It was not achieved before the start of the season and it may not yet be achieved this January. Obviously we need a bigger goalscoring threat than we have got at the moment. That may help us in many ways to achieve more results.
"It certainly will if whoever we get scores goals. But I can't guarantee whoever we get in is going to score goals straight away. I just hope he does [score] when we do."