Key money is not allowed in the Netherlands. It is the amount that you pay to rent the house without something in return. Some departing tenants try to charge this to new tenants. After the rental period key money will not be refunded, in contrast with deposit. If you do have paid key money (and you can proof it), you try to recover this through the court.
You may have paid a fee for special keys with certificate, transmitters of a garage door, gate or similar matters. These fees should be returned to you upon delivery of the house. Key money doesn’t include agency fee you paid to your real estate agency or intermediary.
Key money doesn’t refer to the costs that you make if you broke or lost your key.