When you raise a invoice in a different currency in OpenCRM, Sage Link will pull down the values for the relevant currency, and push this in to Sage.
Providing your Sage company is set with the same default currency there are no problems here.
If the Sage company is set with a different currency to that which was invoiced in OpenCRM, then you will get incorrect figures as Sage will assume that the figures it receives are in the currency for which the company is set up in Sage.
For this reason, the default currency set on the company in OpenCRM should match that setup in Sage. The default currency from a company is carried through to any invoices created for this company, so, by making sure this is set correctly on the company, it leaves less margin for error at the stage of invoicing.
If the company record is created in Sage as part of the Sage Link process (i.e. a new customer invoiced for the first time), the customer will be setup in Sage with the currency passed in from the default currency set in OpenCRM as the currency to be used in Sage, so you should make sure this field is set correctly when creating new records in OpenCRM.