Just so that I understand:
You own x.be
top.be is pointing their DNS to your domain ?? Or their DNS server is listed as being your domain?
If the latter, you would have to setup a DNS server at your domain and then redirect traffic as you thought appropriate.
If they are actually pointing their domain to your host ip in their DNS, then you should be able to respond to THEIR domain by creating a virtual host within your web server configuration file. You would add a virtual host with their domain name on your server, and have it respond with HTML data from a specific directory, etc. It depends whether your server is using Apache or IIS.
Rob