From version 6.3, we now free the unused tester slots, when the study finishes. We still make sure that when a study is being run, you can get a specified number of testers. That's why we lock the requested tester slots. Now, if you change your mind and finish the study before all participants do the eye-tracking test, you can get that spots back in the pool (monthly usage) - and use them in another study.
Please be aware that when you're requesting RealEye panelists and we've already ordered them for you, then you will not get that spots back.
RealEye 6.3 release notes: