X-Cart void or Partial payment refund

I wish to be able to ADD a negative order, for example a customer bought $100 of goods, but one item worth $10 was refunded while the rest was shipped. I should be able to ADD an order to the system for -$10, this keeps the figures in the cart system correct, the customer recieves an reciept for -$10 too. It's such a simple way of doing things, and I have never understood why x-cart doesn't allow this. It's a massive oversight for such a simple fix. This must work with the current version of X-Cart 4.1.4. Coding will have to be in .exe format once complete or something similar that can be uploaded as a simple mod. Coder can have access to my FTP + Database should that be required.