"If they agree in writing then you'll have to go to the conference." or you can still quit

Note: If the OP ends up having to give a total for the reimbursement of costs to you and your friends, I'd include some additional amount for the inconvenience to your friends, potentially up to the total of paying their salaries for the entire alternate time they will now need to arrange to take off. After all, the company is asking them to change their time off too, which may be difficult or impossible for them to coordinate with their employers. The most likely thing is that the company won't even ask for numbers, as they will realize it's prohibitively expensive, just from the situation.

@njzk2 If the company agrees to reimburse for the vacation after he asks for it, then he'd have no choice but to accept. That is only if he asks for it.

Mark you, though, some HR policies let managers do their stupid things and then rebuke them after the fact.The principle is that the mangers are fully responsible of their decisions and consequences. This might be the case after HR have checked that the company, valuing the employee, cannot offer alternatives outside the scope of the current manager. According to the legislation of the case, the employer can be entitled to a severance pay, being none of his/her fault -- so I would not quit, although feeling like that is an important signal for yourself. Just another scenario.

@Makyen that's kind of out of the scope of reasonable expenses and they are things the company probably couldn't treat as expenses from a tax perspective. Notwithstanding, perhaps the OP would be right to ask for a holiday cancellation bonus to share with their friends, but then again it may make them appear unreasonable. Or maybe it's a good idea because the OP doesn't want them to accept anyway. Perhaps someone could dig deeper in an answer.

@NathanCooper "If they agree in writing then you'll have to go to the conference. The chances of this happening are approximately zero. " That's the whole point of this answer, HR will never accept the full cost of OP cancelling their trip.

I'm not sure I'd provide the cost of the trip and give the company the option to buy you out. Presumably you want to go on that trip, and getting a refund for it doesn't solve that. You're giving them an out, that you don't want them to take.

"BTW. Anyone they try to hire should be informed about the situation." Can you clarify this? Are you suggesting that you should tell candidates for employment at your company about the company's behaviour in this matter? That does not seem like a good idea.

@JohnHamilton: If OP's company agrees, that is not yet a done deal. It's purely a willingness to make the deal. OP can still quit their job. Nothing about this would somehow force OP to remain working there. An issue would arise if OP receives the money and then promptly resigns to have the vacation anyway.

"She's just a manager ... you are reasonably safe" - many people leave jobs because of their managers. If you're not on good terms with your unreasonable manager, you're quite far from safe. Although that's not to say I disagree with anything else written in the answer (at least given that OP is willing to quit and there's someone higher up who's presumably on their side).

I would never tell an employer "I'm willing to leave", this can trigger them starting to replace you. If asked about the conference I'd maintain the position that I'm not going, and make no kind of contribution to organising the conference trip, and keep looking for a new job at least until the company backs down on the conference. Maybe even beyond that, depending on how sour the manager gets over this.

The manager has started out with an unnecessary ultimatum threatening termination simply as a dominance display to subjugate the employee over an unimportant matter (as evidenced by the previous manager's approval). This is immediately creating a hostile work environment. Unless the employee submits, staying likely means bad reviews regardless of exemplary work as retribution for fighting back (insubordinate disrespect of authority). This position is untenable regardless of whether HR says they can't be fired over this.

@pluckedkiwi: if the questioner agrees with that, then their strategy should probably be to get the new manager fired ASAP, and then quit if that fails. If the questioner believes in redemption, second chances, or any of that claptrap, then they could take the view that the situation is retrievable if the new manager sees the error of her ways and stops using last-resort threats as her opening position :-)

@RevetahwsaysReinstateMonica Isn't that an issue with the question rather than an answer?

@JollyJoker For sure! I commented in the wrong place. My downvote was on the question though.

@LightnessRacesWithMonica that just means that the monetary reimbursement has to be high enough that the OP and their friends feel that it adequately reimburses them for the inconvenience of rescheduling. But yeah, never make an offer that you're not prepared to follow through with.

This answer strongly implies that there are some official rules somewhere governing this sort of thing. 'If you submit the total cost to HR they have to either approve it or deny it, and if they approve it and pay you, then you have to go to the conference'. None of that is true though. HR could offer to pay the costs and you could still say no. HR could refuse to pay and you could still eat the cost and attend the conference. Or you could quit without talking to HR. Or you could remain employed and simply not show up on the days you had previously requested off.

-1: "If they agree in writing then you'll have to go to the conference" is completely wrong. You are not required to do anything if they say they'll cover all the costs. It just shows that they really want you to go.

@UKMonkey If OP offers to let the company reimburse the expenses for rescheduling and then not go even if they are willing to give that reimbursment I would argue that OP is negotiating in bad faith.

Also, any formal or otherwise noticeable communication with the former manager about this is a very bad idea. An informal communication is OK, but if it's found out they need to be able to say that the meeting was about something else and that they did not bring up the subject intentionally, or at the very least that they were merely seeking "advice". After a manager has given you a direct order, going to another manager outside of the chain of command for relief/assistance will be seen as blatant insubordination.