Seems to me that the airline is being correct and reasonable about the document you've obtained. The second ticket cost you exactly what they say it did. When you combine the receipt for the second ticket with the first receipt (which you haven't mentioned in this post) you'll have two documents totalling the right amount. Even the stupidest accounting department should be happy with that.

The two receipts should reflect the truth about your TWO (not one) transactions. I doubt it will be difficult to get these two documents from the airline.