Cancelled AP Invoice credits closed PA project

We have a situation: an invoice was charged to project 'a' in 2014, project 'a' was closed May 2015, the invoice was cancelled Nov 2015 creating a credit distribution line for closed project 'a'. The cancelled invoice was validated and accounting was created, and the credit line was interfaced to the closed project 'a'.

Because the project is in closed status, should the AP credit distribution line been created to closed project 'a', allowed to validate a closed projects, or interface to a closed project.

Please help me understand why this may have happened.

Is there a way to fix this credit to the closed project?