Expense Gray Areas: What You Can Keep, What Crosses the Line

The money questions nobody answers straight — the per diem you didn’t spend, the miles, the cheaper flight, the personal day. What’s legitimately yours, and what’s expense fraud, told plainly and on your side.

The expense questions people are almost too sheepish to ask — because no vendor and no affiliate will answer them honestly. Here’s the straight version: what’s legitimately yours to keep, what’s a gray area to check your policy on, and what crosses into fraud. On your side, which sometimes means telling you exactly where the line is.

General information — rules and policies change. Confirm current specifics with the relevant authority (your company, the airline, the government) or a professional. Where a public rule applies, its primary source is linked.

Is it stealing to keep leftover per diem money if I eat cheap?

No — that’s how a per diem works. A per diem is a flat daily allowance, not a reimbursement of receipts, so if the company gives you a set amount for meals and you spend less, the difference is generally yours to keep and you don’t owe receipts for it. It’s different from an actual-expense plan, where you’re paid back only what you spent and must return any unused advance. Know which one you’re on — but if it’s a true per diem, eating cheap and keeping the rest isn’t stealing, it’s the deal. Source: IRS Pub. 463 — per diem & accountable plans ↗.

Can I book a cheaper flight myself and keep the difference?

No — if the company books the flight or reimburses what you actually pay, the savings are theirs, not yours; claiming a higher price than you paid and pocketing the gap is expense fraud, and it’s exactly what T&E systems are built to catch. If you found a cheaper fare, the honest move is to book it and let the company keep the savings. The only version where keeping the difference is legitimate is a flat travel stipend, which is rare — if you’re in Concur reimbursing actuals, you’re not on one.

Why am I forced to use Concur when I can book it cheaper myself?

Because the company isn’t only buying the cheapest fare — it’s buying visibility, negotiated rates, duty of care (knowing where you are if something goes wrong), and policy control. You can often find a lower headline price outside the tool; the company is trading that for data and safety it values more. It’s a real frustration, and a fair one to raise with your travel manager — but it’s policy, not someone being lazy.

Can I expense alcohol on a business trip?

It depends entirely on your company’s policy — and this is one to check before the dinner, not after. Some policies allow reasonable alcohol with a client meal, some cap it, and plenty exclude it from per diem or bar it outright. When in doubt, put it on your own tab or ask first; an expensed bar bill that breaks policy is a bad look for a small amount of money. If it’s a client dinner, confirm what’s covered before you pick up the check.

Is it okay to stay with a friend and still claim the hotel money?

Not if you’re on actual-expense reimbursement — you can only claim what you actually spent, so billing the company for a hotel you didn’t pay for is fraud, full stop. If you’re on a flat allowance that includes lodging (uncommon for US domestic trips, which usually reimburse the real hotel cost), that allowance may be yours regardless of where you sleep. The clean rule: know your plan, and never submit a claim for a cost you didn’t incur.

Can I add a personal day to a work trip and still expense the flight?

Usually yes — the company pays for the flight it would have bought for the business trip anyway, and you cover the incremental personal costs: the extra hotel nights, the meals, and any fare increase from your changed dates. The test finance applies is whether the cost would have existed without the personal time — the business flight passes, the extra Saturday night doesn’t. Get it approved in writing first, and book so the business portion is clearly separable. See: blending personal time into a work trip.

Who owns the frequent flyer miles and points from my work trips?

It depends on your policy, and companies split on this. Airline and hotel loyalty miles you earn traveling for work are, at most companies, yours to keep — a common and accepted perk. Credit-card points are murkier: points on a corporate card typically belong to the company; points on your personal card that you expense are usually yours. There’s no single federal rule, so read your travel and card policy — but for airline miles specifically, keeping them is the norm.

Will my company see exactly what I spent?

Yes — assume full visibility. Corporate cards feed transactions straight to finance, tools like Concur itemize every line, and expense reports get reviewed and audited. That’s not a reason for paranoia if you’re playing it straight; it’s a reason to keep clean, itemized receipts and never submit anything you couldn’t explain out loud. The travelers who get burned aren’t the ones who spend a lot — they’re the ones who fudge a little.

Is it unethical to use company-earned miles for a personal vacation?

If your policy lets you keep the miles — and most do for airline loyalty programs — then using them for a personal trip is the whole point of the perk, and there’s nothing unethical about it. The only gray area is if your policy says the miles belong to the company and you use them anyway. So the ethics come down to one question you can settle in five minutes: what does your travel policy actually say about loyalty points? Read it, and travel with a clear conscience.

More answers for the road in the Field Guide.