My suggestion is that you can add 'Paylater' option for that FULL event so that when a new registrant registers for an event, he can select this option instead of paying directly when registering.
Then he will be added in the 'Waiting' list that you can see in the 'Records'. Later, if someone cancels the registration for this event, you as admin can move these waiting lists and send them email for payment. Does this work the same on your site now?