accept negative inventory on sales receipt

scenerio:  Customer places an order online.  She emails me that she's returning her yellow widget and wants the blue one.  She's ordered the blue widget online, along with 6 rainbow widgets.  She tells me to deduct the price of the yellow widget when I receive it, and then ship the new order.

my problem:  Sales order must be converted to invoice.  Credit Memo must be placed for return.  Customer payment must be accepted.  3 steps, 3 sheets of paper to put into customers shipment.

solution:  Sales receipts should allow a negative inventory entry.  Wha-la!!  3 steps combined into 1.  What a time-saving idea!

But this is not possible the way the current sales receipts are set up?