OM - "Return for Credit Only" -"Quantity cannot be greater than original ordered quantity."

Hi,
We are working on 11.5.10  product version and we have following problem:
 
1. Sales order  :
Item:A
Ordered quantity: 1
Unit selling price (USP) = 1000 EUR

Ordered quantity= Shipped quantity
We shipped 1 pcs of item A and invoiced  customer  for  1000 EUR.

2.Customer requires "Return for Credit Only" 

I created  "Return for Credit Only" amounting to 100 EUR , quantity:1 pcs.
When I try to book  another RMA  for credit only for another 100 EUR , I received the following message:
"Quantity cannot be greater than original ordered quantity."
Is there any solution to this problem, since this is not "Return for Credit with Receipt"  and I am not trying to return any quantity, just to correct invoice amount ?