Manchester United have refuted claims by Lazio that they had settled all monies from the £16 million sale of Jaap Stam 16 months ago.
United claim Lazio still owe them £12 million but "remain hopeful" of a satisfactory outcome.
The Dutch defender was sold by manager Alex Ferguson after controversial comments in his autobiography and a loss of form.
Lazio were put up for sale last month but general director Massimo Cragnotti told the club's official website that all debts had been cleared.
"I repeat that Lazio is healthy and has respected all the agreements it had with other clubs such as Manchester United, Valencia and Velez," he said.
Lazio were forced to sell two of their biggest assets over the summer - Argentina striker Hernan Crespo and defensive stalwart Alessandro Nesta - in a bid to balance the books.