Hi Vassilis,
What is the type of the Customer parameter? If it is Static and if you haven't appended the database values into it, it will be empty.
What you're trying to achieve is little difficult and would requrie some serious work either using Command Objects or Business View Manager.
Now, I'm not sure how SAP B1 works and I'm also not sure how it handles dynamic cascading parameters created in Business View Manager (this is a client tool available with Business Objects Enterprise).
If SAP B1 does support dynamic parameters (the ones created using Business View Manager) then please have a look at this post on how to achieve this:
http://scn.sap.com/thread/3212503
-Abhilash