I'm building an online shop using Magento framework which will provide part exchange sales. Customer will select a product from catalog and finally he will place an order. Shop manager will go thrugh the order details and contact the customer to complete the order. Customer will decide to give back his old product as part - exchange for the order. We need to inform customer on invoice that part exchange took place and his order was discounted by eg. USD 100.
Best way of doing this is product with custom and negative price, but this is not allowed in Magento.
Is there any other way to achieve that ?