Can you check that your setup of the item BM500G in the stock modification screen is set up to use 2 decimal places?
There are several places where you choose how many decimal places - each currency also has a number of decimal places it will show.
I feel as though you must be using an older version of the ConfirmDispatchInvoice.php script pre version 4694
You can look at the script version if you open it up in an editor on the first line after <?php
Code:
/* $Id: ConfirmDispatch_Invoice.php 6338 2013-09-28 05:10:46Z daintree $*/