Case of USD — "United States Dollar" or "United States dollar"

According to this Wiki talk page, the ISO standard for currencies (ISO 4217) uses the capitalised versions, but major style guides recommend lowercase. Unfortunately the relevant original sources are not freely available online — although the CMoS (relevant chapter) gives free 30-day trial subscriptions.

General usage strongly favours the lowercase version, as the style guides would suggest, and as a Google Ngrams search confirms; it also shows that US dollar, US Dollar are far more common than United States dollar, United States Dollar:

(Searches with other currencies — UK pound/Pound, Canadian dollar/Dollar, etc. — show similar preference for lowercase.)

Not only is there not much standardisation in this area, there are usages that seem at first blush to be incorrect on some level, but are widely used and understood.

For example "British Pound" is not the "correct" term for the currency, which is called "Pound Sterling" or "Sterling". "British Pound" also does not generate the initialisation "GBP". Despite lacking these merits, everyone understands what the phrase means.

My point here is that as long as your terminology is comprehensible and fairly common, you're probably fine, unless you're drafting a legal document. In that case it is usual to include a definition for any currencies mentioned.