A friend of mine had a similar situation with a German University. In his case it was 5000 Euro. He was also waiting for other universities he was hoping to get into. In the end he paid and couldn't get it back as he got into a better university and attended that one instead.
The rationale behind universities doing this is because they're scared of many admitted students choosing to attend a different university and this is their way of trying to force them into a binding decision.
So in the end its a gamble, don't pay and hope to get in elsewhere, or play it safe and pay and then either lose the money or give up on other unis.