I originally posted this on my employer’s blog, BDO Insights last year. I was asked recently again on using different currencies in Dynamics 365 worked. So here again is a quick guide of how Dynamics 365 money fields, currency lookups and exchange rates work. This will apply to both Dynamics 365 Online and Onpremise.
When a Dynamics 365 org is provisioned, either online or onpremise, a “base” currency is chosen. This usually reflects the country the organization does business in.
If other currencies are used, they need to setup. In Dynamics 365, go to Settings -> Business Management -> Currencies to add the various currencies required by the business.

There is a listing of standard currency codes, you can configure the precision, name, symbol and enter the current or desired exchange rate. Note that out of the box, you need to manually maintain and update this rate.

When you add a money field to an entity in Dyn365, it will automatically add a Currency lookup field and a read-only exchange rate field and a read-only “Base” currency field.

In our example, we have setup an “Agreement” entity with a lookup to contacts and a compensation value.
Dyn365 is setup with a $US base currency. Jim Glynn is paid in $US, so the $US value is entered and you see the Currency lookup is defaulted to US, the base is in US and the exchange rate is 1.

Robert Lyon is paid in Canadian. CRM is still base currency of US. The currency lookup has been set to Canadian, the base is in US (read only) and you see the exchange rate (read only).

Paul Cannon is paid in Euros. Dyn365 is still base currency of US. The currency lookup has been set to Euros, the base is in US (read only) and she the exchange rate (read only).

If you create a new record, the Currency lookup will default to the base currency:

Unless you set your Personal Options to use something else as a default, but a user can still change it when creating records:

NOTE: The base value and exchange rate will NOT change on a record UNLESS the money value is changed on ACTIVE records. INACTIVE records will not change values.
Take care when creating views, rollup fields, reports, etc. that rely on the money fields, mixing currency values tend to drive accountant types crazy!
Hope this helps!
Cheers
Nick