。
I'd like to exchange some money.
Literal
Money-exchange want-to-do [explanatory-んです] [but-が].
~たいんですが is an extremely natural way to make polite requests: ~たい (want to) + んです (explanatory background) + が (trailing 'but' that softens by leaving the request implicit). The trailing が means 'I'd like to, but...(could you help?)' — the actual request is unstated.