Business lunch etiquette

I think there's a very good chance that somebody not involved with this lunch was asked to split up the bill, and didn't know you weren't there. The chances of being intentionally asked to pitch in on a meal you weren't at are seem lower than the chances of a mistake. I draw your attention to Hanlon's Razor: "Never attribute to malice that which is adequately explained by stupidity." If I were you, I'd take it to Boss or shoot off a quick email and say "Hey, boss, I think there's been a mistake here. I wasn't even at that lunch! Lulz. Thanks, anonymous."

That's the most bizarre thing I've ever heard. For starters? This is a person being hosted while they are in town to guest lecture. So who pays for their meals and entertainment? The institution. Does your boss have literally zero expense account? That aside... Unfortunately you can't do #2, which is the correct act of hostility. But it's just too outrageous, and in the end it would make you look bad. I think we're all voting for #4: "Oh, sorry for the mixup, I wasn't actually able to attend that lunch!" The follow-up should be... interesting. (That's when you can start ignoring any other request for money.)

This has happened to me as a grad student - I got an email from a faculty member asking to split the dinner bill for a guest speaker from a few weeks before. I emailed him back to say I hadn't been at the dinner. Turns out he just wasn't able to remember who had actually attended, and emailed anyone who was likely to have been there, by virtue of research overlap with the guest. You should be fine emailing back and saying you weren't at the lunch.

You volunteered to pay for dinner, not lunch. I would go with the "there must be some mistake" tactic as above. But at a dinner for around six people? I would be a little upset that your boss couldn't remember you weren't there anyway.

So who pays for their meals and entertainment? The institution. Does your boss have literally zero expense account? Zero expense account is mostly true at my institution. Our department can just about pay for a guest's meals, but anyone who goes out to lunch or dinner with the guest is on the hook for their own share of the bill. It's the norm for any of us who take a guest speaker out to dinner to deal with splitting up the bill later (minus the portion that the department is covering for the guest). I'm not sure if this is the case at the OP's institution as well, but it happens.

(You might want to quickly check in with a friendly department secretary or whatever admin works on your program about whether you are really expected to chip in and whether it could actually be covered by your program - as a secretary, I would never want a student or postdoc to be out of pocket on any guest expenses and have advocated for reimbursements for people in the past. It might be that there is money for this stuff, but there has been some miscommunication or funny business...And of course, don't pay for the lunch! As far as I'm concerned, you really shouldn't have had to pay for the dinner either; it's exploitative and unprofessional for older, better-compensated academics to let younger, less well-compensated folks pick up a tab like that, although it was nice of you to offer. )

I think the rule at most institutions I was at is that the guest is covered, and either none or a set amount for anyone else is covered (it varied) so this isn't totally atypical. Anon, are you new to your institution? I'm wondering if perhaps the reason this wasn't discussed is because it's some sort of casual rule. This actually happened to me before. I let the person (an administrator) know that I didn't know about it and that I felt uncomfortable being asked to cover expenses for an event I didn't attend, and that was pretty much that. (I will note that this irked the administrator, for reasons that had little to do with the amount or my approach, and everything to do with me being a young person refusing to obey my elders. But I digressed.) At any rate, I'd definitely question it. You already paid for dinner, you did not attend the lunch, and you didn't know you'd be on the hook for any portion of it. I'd be interested in a follow-up on this, Anon.

If you want to soften it a little, follow up the excellent first few suggestions of "but I wasn't there" with a sentence asking (since Boss paid) "do I make the check out to Boss or to Boss's department?" This can let you lead into "If I'm reimbursing the department, I need to submit my receipt for dinner the night before so everyone can pitch in to reimburse the department for their share" etc.

How about none of the above? Go ask your boss what's up. You're a professional and this is a business relationship. If your boss is playing games, well, I think you'd know up-front whether he's the type. But if you don't then I guess you can make your own decision about whether you're willing/have to play along with coy nonsense. I made the decision long ago that I wasn't going to engage in nonsense or be mistreated at work but I'm not under the illusion that other people don't necessarily have that liberty. But I don't see any reason why, in an office that isn't dysfunctional, you can't simply walk into the boss's office and say "Hey boss, I'm confused by this request."