If balance forward is a summary field, why is there a "c_" at the beginning? You can name fields however you want, but most naming conventions would use a c_ like that to identify a calculation field and s_ to identify summary fields so I am checking to see if this is really a summary field.
If it's really a summary field, you can add a calculation field to Invoices that copies the value of the summary field (To thus get a total for that invoie). Then, if you add a one row portal to invoices and specify that the sort order for the invoice places the newest invoice first, you can put this calculation field from invoices to show the balance forward for just the most recent invoice.
You should be able to sort your portal records by either a serial number or a date field in descending order to put the newest invoice first.
Thank you Phil, as always you get me unstuck! Balance Forward is a summary field and I should have named it using s. Thanks for pointing this out too!