Lukaku moved to Inter in the summer of 2019 for a deal worth a reported €80m.
Corriere dello Sport now claim that the deal included a clause that meant if Inter missed any payment deadline they would immediately have to pay United the remaining amount.
The report alleges that the Italian club have missed one of those payment deadlines in the midst of financial issues and are now in €50m (£43m) worth of debt to United.
