FIFA has confirmed it is investigating a claim from Sampdoria against Sporting Lisbon in relation to the "financial obligations set out in the contract corresponding to the transfer" of Bruno Fernandes to Manchester United.
When Sampdoria sold Fernandes to Sporting Lisbon in 2017, they inserted a 10 per cent sell-on clause on any profit Sporting made on the player.
Sporting sold Fernandes to Manchester United in January for an initial €55m (£48m), and Sampdoria believe they are owed €4.6m (£4m) as part of that deal.
But Sporting believe the sell-on clause is null and void as Fernandes had signed a new contract with them in July 2018.
Fernandes was one of seven players who terminated their contract with Sporting in response to an attack by fans at the club's training ground in June 2018 - and Sporting argue he signed a new deal with them as a free agent.
A FIFA spokesperson has confirmed a complaint was lodged by Sampdoria on April 3 and that "the matter is currently being investigated and consequently we cannot provide further comments."