I've seen this kind of nonsense before, and honestly it pisses me off. If these fees aren't listed in the initial quote or in the agreement they proposed to you, you can simply refuse to pay them - they can't just force them on you after the fact. The rule is that all fees must be clearly stated before you commit to anything, not on signing day.
Get them to send you a detailed explanation by email of what these 500€ represent and ask them to show you where it's written in the documents they gave you at the start. Nine times out of ten they'll admit it's not in there, and you can push back without feeling guilty. If they really insist, you can also consult someone who knows Belgian property law well to verify you're covered, but in most cases you have the right to say no.