If the booking requires a credit in order to balance a booking because it has been short paid and the booking should be released.
Goto the Booking and select (+), then select 'Fees', in this sections you'll see the option 'Add Transaction' select that option. In the 'Add Transaction' screen select the Transaction type 'Booking Credit' (there are other types including Booking Credit MF for adding a management fee for the credit).
Once that's done, go to the Account dropdown and select 'Non-Account' ** This is very important