A good example of non-negotiable, while traveling overseas, for one particular company, I was just randomly doing an expense report - and noticed there was a "medical" line-item in my expense report, that I figured I would use for cough-drops that I had purchased. Finance of course went ballistic, because the policy against expensing anything with that line item was absolutely forbidden (which begs the question as to why it was available in the first place).

Here is the thing - the company would have zero difficulty with me asking for a $100/day per diem while traveling for months on end, instead of doing expenses - that was negotiable. But there was absolutely zero chance of getting a $7 bag of cough-drops allowed as a medical-expense. Non-negotiable.