title: Magento Credit memo, changing ORDER-id to INVOICE-id
body: <p>In Magento you can cancel an order or invoice, and set it to a creditmemo. This is working correctly, but in our case we need the INVOICE-id, instead of an ORDER-id. By default, Magento uses the INVOICE-id.</p>
  
  <p>This isn't some "setting", but it would probably need to be coded by hand. That's not a problem for me, but I can't seem to find the right file to edit. </p>
  
  <p>Does anyone know whether this is even possible? Or does someone else had this problem before? </p>
  
  <p>Thanks in advance!</p>
  
  <p>:( Can't seem to set a bounty, otherwise i would have done it.</p>