Xero Multi-Currency is enabled for Xero subscriptions that pay the small increase for Multi-Currency.
Salesforce Multi-Currency is enabled by filing a case with Salesforce Support and asking them to turn on Multi-Currency. It is available on all Salesforce editions. Because Multi-Currency cannot be uninstalled or turned-off once enabled by Salesforce, we strongly recommend only turning on Salesforce Multi-currency if you are absolutely sure you will need it. We must emphasize this - there is no way to ask Salesforce to turn off Salesforce Multi-Currency, so test thoroughly in your Sandbox first.
Breadwinner and Xero Multi-Currency
Breadwinner works with Xero Multi-Currency. Actually, whether you have Xero Multi-Currency or Xero Single-Currency, Breadwinner will operate the same. All Breadwinner Invoice records in Salesforce have the Xero Currency and Xero Currency Rate field on them.
Using this field, you can run reports displaying the currency code. With Summary Reports that are grouped by the Currency, you can create reports with per-currency subtotals. With formulas, you can create additional fields displaying the exchange rate. The possibilities are endless.
Default Currency for Xero Orgs and Xero Customers
When you create an Invoice in Xero through Breadwinner, you do not need to set a currency. This is because Xero will create the invoice in the customer's default currency (set on the Customer Edit page in Xero). If you haven't set a default currency for that customer, Xero will create the invoice in the Base Currency for that Xero org.
If you need to override this on an individual invoice, once you create the Invoice, edit it through the Xero Web Interface and change the Draft Invoice to the currency of your choice.
Breadwinner with Salesforce Multi-Currency
Currently, Breadwinner is not aware of Salesforce Multi-Currency. What that means is that Breadwinner will work the same whether or not you have multi-currency enabled at the Salesforce level. While enabling multi-currency may provide additional features you desire, such as price-books that have multi-currency enabled, we will still pull data from the same fields, and will not know that those fields may or may not be multi-currency enabled.
Given this, if you haven't turned on Salesforce Multi-Currency and are considering doing so, we strongly recommend you investigate all possible implications in your Salesforce Sandbox. When testing this, you can connect your Salesforce Sandbox to a different Xero org, instead of your production Xero org.
You can get a free 'Test Org' from Xero by going to My Xero, and clicking (slightly hidden towards the bottom of the page) something like "Try the Demo Company...". Please note, we do not offer support on starting a Xero Demo org, please contact your accountant or Xero for assistance with that, or read the Xero Help Articles on a Demo Org.