Another VL problem

[gaby78]

I have an Invoice table and a Payment table with a join table in the middle:

INVOICE ------ PAYMTDETAILS ------ PAYMENT.

In the PaymtDetails portal on Payment layout I have InvID set as drop down list based on a value list. The VL is built on a second TO of Invoices linked to Payment by custmer ID (gCustID in Paymt = CustID in Invoices2) to allow me to filter the VL by customer. It is working fine. I tried to improve on this by wanting to further filter the VL to show only invoices that have a balance left (gCustID = CustID AND gAmount in Payment < InvBalance in Invoice). InvBal is an unstorable calc field in Invoices. The VL stopped working because InvBal is an unstorable calc field. How can I solve my problem? How can I filter my VL by customer and show only Invoices with a balance > 0. I tried a nbr of things with no results...can't figure it out.....my head is spinning.

Thanks for any help