Hello Dror,
1.Go to Customers work center->Account view->Open an Account e.g. "ABC" and go to the Sales Data facet->Click on the New button ->In the New Sales Data window maintain the Currency e.g. "USD" for the combination of Sales Organization and Distribution Channel and Save it.
2.Go to Sales work center->Create New Opportunity for account "ABC" and same combination of the
Sales Org and Distribution channel (maintain in Step 1) .
=>You will see , the currency which is defaulted should be "USD" (PS: Can see fro e.g. under field 'Expected Value' within opportunity Header level).
Hope this explains now.
Best Regards,
Ankush Pashine.
