Last year I opted to fly me-paid business instead of company-paid economy on one of my trips. I purchased the business class ticket, and at the same time printed off what the economy ticket would have cost through the same booking channel (direct with the airline) and sent both the ticket and the print off to accounting, claiming the amount for economy on the expense voucher that accompanied it. My accounting department were ok with that, even though it was not a "receipt" for the correct amount, which is what our expense policy says. I would chat to someone in your accounting department and work with them on how to do this in a way that satisfies everyone, including the tax man!