On the ticket there should be a series of codes and prices, with some guesses I think you can figure out. Possibly also on the receipt. -- But you may ask your finance department or other employees: I'm sure you are not the only one with such problem (and so I expect an internal guideline for that, for sure hidden)

Airline pricing is tricky. Booking multiple legs and return flights affects the total pricing. I don't think you can get an "itemized" receipt you're looking for.

There’s a good chance that this return ticket is actually cheaper than a one-way ticket…

Guest really has no reason to be angry at anyone than themselves. They either fix what they messed up or end up without a refund. Getting proper receipts for whatever you want reimbursed and not mixing it with private expenses is like Travel Expenses 101.

Also, if you invited a guest, is it not normal procedure to refund the flight both ways?

@jcaron The other flight is more than three months before the relevant one,so presumably an unrelated trip. I would assume that the other direction was on a different booking which is also getting refunded, but it would be great if OP could clarify.

@TooTea There are perfectly legitimate reasons to need to split up a flight receipt, however. I've scheduled interviews with two employers in the same distant city for consecutive days, and had the two companies each reimburse half for the flight, which would have been all but impossible to split 50-50 with separate bookings. It might also have been cheaper to book the round trip instead of two one-way flights. I'd also be a bit annoyed if I booked the flight in the most sensible and cost-effective manner and got the runaround from the bean-counters.

The guest bought a round-trip ticket. Was your original intent to not reimburse for a round-trip? A one-way trip reimbursement policy sounds extremely anti-guest.

My experience in comparable situations is that the reimbursing party would normally calculate what the price of the agreed tickets would have been, and reimburse that. You cannot get a receipt from the airline for that price, so if you want any reimbursement to happen someone will need to argue about the documentation requirements. My experience is with reimbursement of employee business trips by US companies; the standards of evidence involved are up to the company, plus whatever the IRS wants as evidence for tax-exemption.

@MonkeyZeus no they didnt book a round trip in the sense you imply as the question clearly shows and as has been discussed in the comments above yours.

What instructions did you give the guest about booking the tickets? Is your organisation in DC or Venice, i.e. did they arrive really early for your purpose, or "leave" a few months before "coming back"?

@MonkeyZeus a round trip doesn't have the return before the outbound flight. 3 months earlier.

@DonQuiKong You're putting a lot of stock into forcing the guest to fly within a specific date range. OP verified that the guest flew from D.C. to Venice on November 10th. Assuming that the guest fulfilled their length of stay (let's assume 2 days) then it shouldn't matter the date of their return flight. Would you have penalized them for staying an extra two weeks to do some tourism? You'd have to prove that the guest's date range is significantly more expensive than a quick turn-around.

While I don't know the situation now, a decade ago round-trips with "traditional" airlines used to be super expensive when they did not include a weekend (or something like that), so, business trips. A mitigation was to purchase two "long" round trips instead, ABA and BAB, where only the first halves were used, and the throwaway part was on a cheap flight, possibly several months later. You may be looking at something like this now, just the guest presumably wants to actually use all of them. You could reimburse half of the price, and then everybody saved a bit of money.

This is also a life lesson for you and your organisation. If you invite somebody, you (or your secretary or admin) does the booking, not the person you invited. This removes so many issues by design. (another one that I recently have seen is an invited speaker at a conference who got sick the day before the trip, then did the talk remotely, and then was unable to get a refund for his prepaid trip since he wasn't actually physically there)

Sorry everyone, I had some very busy days. To answer a bunch of stuff I've seen here: 1) This is not a company, this is a public institution. It's simply illegal to reimburse a value different from what is specified in the receipt. We cannot "just reimburse 50%". 2) We cannot reimburse a flight took on the 5th of August when the guest participated at our even in November. They simply happened to be in Venice for holiday and decided to take a roundtrip, but we can only consider the flights on the 10th as valid expense for the event.

3) The total amount is over the budget, so even if we wanted to just reimbure in full we couldn't. 4) We asked the guest if we should buy them the tickets, but he insistent on doing it themselves. I could not possibly had idea that they happened to be on holiday in Venice in the summer and planned to book the flights like this making it hard to impossible to legally reimburse.

I don't get it: if you bought them a ticket yourself, would you have bought them a one-way from DC to Venice, rather than a round trip?

@JonathanReez We told them "we are going to reimburse at most X€", they went ahead and spent 2-3 times as much flying business class. We would have bought economy tickets for a roundtrip. This was clearly specified in the first contact we ever had with them, so they knew from the start we would not cover a roundtrip intercontinental business class flight. My problem is that with their receipt we can't even provide any reimbursement for the flight.

Okay so you said you can provide a reimbursement up to $X. Why not just pay them X? My company does that for meals while on business trips - if I spend too much they just reimburse me up to the daily limit.