It is possible to get a closed Invoice with a negative unpaid amount. Such documents have several applied payments which are based on a Payment Method with "Manual" Payment System like "Check/Cash". These payments were imported from an external CSV file as described in OA Billing Provider's Guide.
Behaviour is caused by software-related issue #PBA-81593 that affects only imported payments based on manual payment systems.
Contact your Technical Account Manager or Pooled Technical Associate Team at email@example.com to clarify the status of #PBA-81593.
To fix already affected subscription please contact Odin Technical Support.