Deposit

What does the expert's report say, and have you signed it?

Owner/landlord can demand as much as he thinks will cover the cost of the damage as set out in the etat des lieux.
You can always disagree and get a 2nd opinion for the etat des lieux but that would also be at a cost where the original entry etat des lieux would be compared with the state of the property at exit.

More here:
http://www.belgium.be/en/housing/renting_a_home/

relative page not in English: http://www.belgium.be/fr/logement/location/litiges/