Tottenham are being asked to make a compensation payment by Shakhtar Donetsk after bringing Manor Solomon to the club.
The Israeli forward spent last season on loan at Fulham and was able to make a permanent transfer to Spurs on a free, despite being under contract at his former side.
The deal was able to proceed due to a FIFA ruling that allows any player who is under contract at a Ukrainian club to extend their deals for a further year.
The clause known as Annex 7 meant Solomon was able to extend his deal past its December 31 expiry date and join as a free agent.