Remind user when customer has overpayment

Hi,

I asked this recently, and I thought I found the solution but this was not the case.
We created a workflow where when a new invoice is about to be created, it checks customer balance, and shows a message if it is less than zero.

Turns out this approach is not 100% accurate. Some customers have positive balance, but in their aging we can see other overpayments. e.g. 61-90 days is -$200.
I cannot see any aging fields under customer record when building the conditions with the visual tool. Is it possible to utilize the saved search for this? e.g. I have a saved search that brings the list of customers whose aging showing negative numbers. But simply plugging this search in the condition of the workflow does not work. Actually how search condition works for an event based workflow was not described well.