When you have to pay an amount to be able to rent a property without any performances in return, this is called key money. This is not allowed. It is a so called ‘unreasonable clause’ in the rental contract, which is forbidden by law. If you do decide to pay key moneyn (which we from FMH discourage!), you can reclaim this amount through the courts. You should be able to show you’ve paid this, however, by means of a receipt or witnesses.