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From your screen shot, it appears you want a total of the field Bills::Balance_Due
If you define a field in Patient management as:
Sum ( Bills::Balance_Due )
It will report the total fo that field for all records in the portal, provided that the portal is unfiltered.
You can also define a summary field, sTotalDue as to the total of Balance_Due in the Bills table, then add it to the layout shown and it should report the same exact total.