A member of oscommerce forums noticed that "If you run a multi currency store it doesn't take into account additional currencies." (http://forums.oscommerce.com/index.php?showtopic=139964 ).

To fix that go to catalog/includes/languages/english/product_info.php and Replace:

define('TEXT_FREE_GIFT', 'Free gift with $' . $product_gift['threshold'] . ' purchase.');

With

define('TEXT_FREE_GIFT', 'Free gift with ' . $currencies->display_price($product_gift['threshold']) . ' purchase.');

More...