Magento supports the ability for Merchants to;

1) Set up different websites with different currencies and prices for products

2) Enable different currency display options, where by the customer is still charged in the base currency (eg USD), but can opt to display the prices in another converted currency (eg EUR) on the website.  

Merchants should ensure the currency setup on their Braintree and PayPal accounts is the same as that of the ‘base’ currency as configured in Magento. The merchant is able to change the Braintree API Merchant Account ID on different website levels within Magento to ensure the customers are charged in the correct currency. 

Equally, by utilizing the additional options within Magento to change how the currency is displayed the merchant is able to display multiple currencies but ensure the customer is charged in the base currency configured in their Braintree/PayPal accounts.

For PayPal payments the merchant must ensure they have enabled the desired currencies in their PayPal account - this can be found under the "Currency Management" within the PayPal account management site. If this is not done, PayPal will reject payments with the error message "The currency of this PayPal transaction must match the currency of the merchant account.".