I am trying to add the balance summary for each contact's invoices to the Contact List layout but the result is not unique for each record on the list.

I am using ContactGlobalsInvoices::Summary_TotalBalance as a field on Layout Contact List. Is there a way to make this field reflect each record's summary_totalbalance?

Here's a sample of the modified layout:

If I'm understanding your question correctly, you would create a calculation field in the Contacts table that adds up the totals of all related invoices. Easiest way would probably be to use the Sum() function. Then put that field on the portal shown in your screen shot. HTH -Jeff