The currency exchange support is way simpler than this. The library only provides a method that will calculate the monetary amount given another Money object as the rate:

val amount = 1 money "USD" // USD 1.00

val rate = 5.4905 money "BRL" // BRL 5.4905

amount exchange rate // BRL 5.49

It's up to implementers to hook up in other datasets and to consume the rates. Exchange rates datasets differ from country to country and some foreign exchange companies offer short-term contracts to hold a transaction at a given rate (sort of "guaranteed rate/price"). I don't see myself supporting this use case.

Nevertheless, Java Money (Moneta) has this feature. Never used it so, I don't know how it works.