When a former employer sent me on business trips, the bean counters would complain that my descriptions for the purpose of meals on my expenses were not descriptive enough, as if the purpose of eating was not obvious. I ended up writing something like "nourishment to remain alive while traveling for XYZ project" out of frustration after that. That did the trick and shut them up. I suppose it was hard to argue that description, because if they disputed it, they'd basically be admitting they were sending me away because they wanted me to die.

The trick is to either get an example from them of what they think is a descriptive invoice, or to break down your invoice filling procedure into a step by step workflow or checklist.

Someone isn’t necessarily being an asshole, that person has their own struggles, goals and priorities. Managing a condition and someone else’s expectations or their own condition is the hardest part of working together. This is where soft skills help, too