If you approve a timesheet in error, then you can easily go in and unapprove it afterwards. To do this just click into the approved timesheet, and press the red unapprove button in the top right hand corner. This will remove the sales and purchase invoices (that were automatically created inside Vincere).
NOTE: If you reject a timesheet in error, there is no option to go back in and approve this timesheet. Your candidate will have to go back in to their Pay & Bill account and resubmit.