u/ArnesianOrphans

Hi there ! :)
I’ve been living in Brussels for 2 years and now planning on leaving. On my contract it is said that if I leave I need to pay a full month. But if I find the next people to live in my apartment then I will pay “only” 500€. I heard some people telling me it’s not legal. Do anyone happen to know something about this ? I think 500€ is too much for this. I rent the appartement through an agence.
Thanks :)