Create a new relationship to the ledger which emulates the filter. Leave the filter on the portal the way it is if you want, but have the Sum calculation use this new relationship. In other words your current relationship will be something like gPatientID = PatientID, and the filter is gMonthNumber = MonthNumber in the other table. Have a new relationship which includes both those parameters.
Thanks, sorbsbuster. I managed to stumble upon this fix during the late hours. I created a new relationship between my Invoices table and a duplicate occurance of my Master Ledger Table. I created a calculated "tag" field to distinguish line items (billable sessions) the occurred during the current month (ginvoiceID Month(DOS)&Year(DOS)&Pt#), from line items that occurred during a prior month, but were paid during the current month (gInvoiceIDPrior = "prior"&Month(DOS)&Year(DOS)&Pt#). Doing this , I do not even need to filter the portal. I simply set the portal 1 up to draw related records based on gInvoiceID = InvoiceID, with portal 2 set up for gInvoiceIDPrior=InvoiceIDPrior.
I had tried setting it up like this before, but it did not work for the relationship set up with gInvoiceIDPrior=InvoiceIDPrior. I finally realized that I did not have Indexing truned on for this field!. Once I fixed this, the related fields popped right in to portal 2.