FC Porto are believed to have threatened to report Atletico Madrid to FIFA as they claim money remains outstanding from last summer's €40 million transfer of Radamel Falcao.
Spanish daily Marca has reported that Porto president Jorge Nuno Pinto da Costa was close to making an official complaint that could see FIFA impose a transfer embargo on Atletico.
The Spanish club agreed to pay €40 million for the Colombia striker last July, with the money to be paid in installments: €22 million was paid at the time, €9 million was due by July 2012 and €9 million next year. Porto have only reportedly received €6.5 million of 2012's payment.
Sources at Atletico told Marca that there was money outstanding but it would be paid soon, once the club received money due to them for transfers completed this summer. Atletico have moved on eight players this transfer window, including Argentine winger Eduardo Salvio to Benfica for €10 million and young Spanish defender Alvaro Dominguez to Borussia Moenchengladbach for €8 million.
With Atletico hugely in debt - including a reported €215 million owed in taxes - chief executive Miguel Angel Gil Marin said last spring that Falcao would be sold this summer if the club did not qualify for the Champions League. He then backtracked after the striker's superb double strike saw off Athletic Bilbao 3-0 in last May's Europa League final.
Falcao was accompanied from Porto to Atletico last summer by Portuguese international Ruben Micael. Los Colchoneros reportedly paid €5 million for the midfielder, who spent all last season on loan at Real Zaragoza and has recently joined Portuguese club Braga in another loan move.
Both Falcao and Micael are represented by Portuguese super-agent Jorge Mendes. Persistent rumours in Spain have linked Mendes with a third-party investment fund that helped Atletico sign Falcao and retains some of the player's economic rights. Gil Marin has denied these stories, saying the club own 100% of the Colombian's registration.
Falcao, who scored 36 goals for Atletico last season, and 31 goals the season before for Porto, has previously been linked with many of Europe's top clubs, including Chelsea, Manchester City, Real Madrid and Juventus.