I have just noticed bug number 208.

I had the same problem and I did not trust myself or anyone else involved with processing orders to remember to add the "-" sign, so here is the workaround I put in my files:

Couple of notes first:

1. I am not using RC3 as my modified site means I have more integration to do before I can implement this.
2. For ease of use, I have looked at the default RC3 and referred to it in the following line numbers.
3. I only use Gift Vouchers, if you use others credits such as Discount Coupons or Customer Loyalty Schemes you will have to change the IF statement below or add OR clauses, ie. $ot_title == 'Gift Vouchers:' || $ot_title == 'YOUR OTHER CREDIT:'

Open admin/edit_orders.php
around line 393 find
PHP Code:
 value = '" . $ot_value . "', 
change to
PHP Code:
 value = '" . ($ot_title == 'Gift Vouchers:' ? -$ot_value : $ot_value) . "', 
Open admin/includes/classes/order.php
Note that if you are using an earlier version open admin/includes/classes/oe_order.php
around line 36 find
PHP Code:
 'value' => $totals['value'], 
change to
PHP Code:
 'value' => ($totals['title'] == 'Gift Vouchers:' ? -$totals['value'] : $totals['value']), 
This is only a workaround and I am sure someone with far better coding skills than me will be able to clean it it. However if you need any help with it please drop me a note.