Supporters threw rolls of mock-up €500 bills at the Constant Vanden Stock Stadium in protest against the high cost of tickets.
They also held a banner apparently criticising UEFA for their pricing policy, which read: 'Is your gr€€d now finally satisfied?'.
Travelling fans were angered after some reportedly paid as much as €100 for match tickets.
European football's governing body has decided to fine them after they were charged with throwing objects and displaying illicit banners.
Spartak Moscow have also been fined €3,000 for setting off fireworks in their draw with Maribor.