Core gives you an option to reverse invoices. When reversing invoices, it cancels your billing decisions and releases the WIP (approved time and expenses) back into the database for later billing. After getting reversed, the invoice can be re-processed from the Invoices screen. E.g., a time entry might show up as No Charge on the invoice because it was set to non-billable by mistake. In this case, you can reverse the invoice, make the correction and then re-process the invoice. Check the Core Help for details.