It really depends (I know, that doesn't help much). On our Knights & Lights trip, Adam was British, so I paid him in British pounds. US dollars wouldn't have done him much good. And we knew that Landon was returning to the US after our trip, so I paid him in US dollars. But, normally, I pay in local currency, as they can usually use it or deposit it where they are. If it's US dollars, and they're going to be in Europe for a few months, it could cost them to have it converted.
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