Seems to me if you have to leave a coin (foreign or not) in a cache as an ALR that would be a commercial cache, you have to pay to log.
Commercial Caches
Commercial caches will not be published on geocaching.com without prior approval from Groundspeak. A commercial cache is a geocache listing or geocache which is perceived by Groundspeak, Groundspeak's employees, or the Volunteer Geocache Reviewers as having been submitted to geocaching.com with the principal or substantial intent of soliciting customers or generating commercial gain. The geocache is presumed to be commercial if the finder is required to go inside a business, interact with employees, and/or purchase a product or service, or if the cache listing has overtones of advertising, marketing, or promotion.