In Customer Profile under Balance would like to add a new field for Open Invoices. This field would display the sum of the Current Balance for all Invoices with an Open Status associated with the particular customer. Need help creating the customization.

The issue we are having is that Prepayments are included in the Balance the Customer owes. If a Customer has open Invoices totaling $1000 but they have a Prepayment of $200 for future work their balance displays $800 and this balance is what our employees see and what shows on the Customer Statement. Customers end up paying less than what they owe.