I was recently asked for advice on the "correct" way to write sums of money in euros. The possibilities are: 999 euros, 999 EUR, EUR 999, 999 a or a 999. I personally tend to write things the way
they are spoken (999 euros), which I can do because I usually imagine the texts I write being spoken by somebody, so they're not very formal in style.
This is the main question in so many cases, I think: how formal is the text? In the case of more formal texts like notes to accounts, I would expect abbreviations or symbols to be used, EUR or a
rather than the word "euros". Then I would say that EUR is more formal than a, and the tendency in moving away from speech to the written form is to put the EUR or a before the amount, as with GBP
and , or USD and $, so EUR 999 or a 999. This is advice by analogy rather than rule, though.