Allocate tickets at registration rather than Payment Complete -

If you set a capacity for an event under some circumstances this capacity can exceeded
We experienced a very high demand for some of our events and people were desperate for tickets.
The registration opened at set time and people could see this time, so as soon as ticketing open a large number of people started registering.
The issue arises because, the capacity is not reduced until payment is completed and the user is passed back to Event booking
So let's say there are 2 tickets left and Person A registers for the 2 tickets, He/she then is taken to the payment system (ie PayPal) to pay for the tickets. While that person is paying for the ticket, Person B comes along and will still see 2 tickets still available. They register also for the 2 tickets then go off an pay for the tickets. Person A finishes paying and it allocated the tickets and the capacity is full. Person B then comes back after paying for tickets and they then also get their 2 tickets and the capacity is exceeded by 2 . A screen shot of what happened to us is attached.
So what would be better is for the ticket to be provisionally allocated at registration time not payment completion time. Then if the Payment is not completed the tickets are released back for others to take