Just go back into COAsT and choose your student. Then click on ‘Actions for selected students’ and choose ‘Assign test’ as if you were doing this for the first time again.
When the details come up, before you click on the orange button ‘Assign online tests now’, you will see that you are nor charged any credits for the test/exercise. It should say in the notices section ‘assigned previously-no charge’.
This is only the case for exercises/tests that have previously been assigned and paid for in credits.