West Ham United are trying to resolve an issue that has arisen around the £25m transfer of Algeria winger Said Benrahma from Brentford.
Benrahma, 25, had the first part of his medical on Thursday and the Premier League side were confident of completing a deal on Friday.
However, it is understood there has been a complication, though sources have told the BBC they still expect the transfer to go through.
The transfer deadline is at 17:00 BST.
Benrahma scored 17 goals in 43 Championship appearances last season as Brentford reached the play-off final.
It is understood West Ham have agreed to pay £25m up front for the player, plus £5m in add-ons.
The Bees signed Benrahma from French club Nice for less than £3m two years ago.