Portuguese champions Sporting Lisbon are one of eight clubs that have been sanctioned by UEFA for Financial Fair Play (FFP) breaches.
BBC reports that Sporting has been fined 250,000 euros, while league rivals Porto were fined 300,000 euros.
They have until January 31, 2022, to pay the overdue amounts otherwise face a ban from UEFA competitions.
Sporting have qualified for the knockout stage of this season’s Champions League.
The other six clubs sanctioned are Spain’s Real Betis, Astana of Kazakhstan, Romania’s CFR Cluj, CSKA Sofia of Bulgaria, Gibraltar’s Mons Calpe, and Portugal’s Santa Clara.
UEFA said all eight clubs were sanctioned due to their “non-compliance with the ‘no overdue payables’ requirement”.
A statement from European football’s governing body warned they faced additional punishment if they did not pay the overdue amounts.