I've seen plenty of waiters, taxi drivers, etc., be quite happy to receive tips in USD in many countries where USD is not the official currency. In fact, I can'

I've seen plenty of waiters, taxi drivers, etc., be quite happy to receive tips in USD in many countries where USD is not the official currency. In fact, I can't think of a single time when I've seen such a tip be rejected because of its currency. That's quite different from trying to pay a bill (invoice) in USD in those countries.