Capture balance amount

[Rainy Pass]

How can I capture a current balance in a point in time?

Tables: Customer, Order, Item

Each customer has a starting balance, as they place orders, there balance is reduce by the price of the items.

I need to report monthly, the beginning balance for each month, and show the Items and the ending balance.

I understand how to build a report for all transaction. If I only want to show one month, how do I capture/show the balance at the beginning of a given month?