Symptoms
A customer is able to cancel a PayPal payment after it is actually completed, with the status of PayPal transaction in OBAS changing from Completed to Declined.
Cause
Such behavior was classified as the software issue PBAS-30516 and was fixed in OBAS 4.5.7 release.
Resolution
To resolve the issue, please upgrade to 4.5.7 as described in How to upgrade OBAS system to the latest available version.