In the upcoming release of Fortnox, a new functionality will come where a database will be able to get its exchange rates updated automatically. In order to use the function, the function must be started inside Fortnox’s settings (it does not get started via the API). What calls to make for each function described below can be seen here: https://developer.fortnox.se/documentation/resources/currencies/

When a database has activated automatic exchange rates, all documents submitted without an exchange rate will receive the exchange rate corresponding to the current invoice date for the document, a document were a set exchange rate is sent in the call trumps the course functionality.

If you try to update the exchange rates via a API to a database and currency that has the “automatic exchange rates” setting, it will not work and the following error message will appear: Cannot update currency XXX, it will update automatically.
There is an exception to this, that is when a currency is not provided for automatic courses, then that currency will have a flag that looks like the following:

{
"@url": "https://api.fortnox.se/3/currencies/DOP",
"BuyRate": 5.99,
"Code": "DOP",
"Date": "2018-11-20",
"Description": "Dominikanska pesos",
"SellRate": 0.1669,
"Unit": 1,
"IsAutomatic": false
},

When a currency code is not automatic, it is possible to update the exchange rate via the API.

When a database has the automatic exchange rates setting you will always get the latest available exchange rates through the API.
Fetching the currency through the API does not update the currency rate. This is done automatically once per day between 12:00 and 13:00.