The currency is sometimes considered a proper noun, the individual denominations are always considered common nouns.
if I'm working on the stock exchange I might track the value of the Dollar (a proper noun). If I'm buying from your lemonade stand I'd give you a dollar (a common noun).
likewise the Ouguiya is worth about 1/400th of the US Dollar. If you go to the bank, they could give you a 1 ouguiya coin.
Here's a source and here is the MLA style guide not capitalizing dollar.