Here's the idea, please tell me if the premium version of your plugin can do it (since the free version can't import to users):
-have a list of users
-keep accounting records in QuickBooks (obviously not synced to WordPress)
-export users' records and their current balance due once per week/month from QB to CSV
-import from CSV to EXISTING users' profile field (a custom field called 'currentbalancedue', for example)
-then, instead of calling for their balance, they could just login to their account and see one of their profile or other fields that says Current Balance: $xxx.xx
Again, this is for updating existing WP users, not one-time creating new users.