"Make all checks payable to the Gamma Sector"
Of course, if they're asking for their own currency, you can claim any exchange rate you want. "Here's fifty oak leaves, that should be worth 3,000 Zipwhals. Great doing business with you, although I don't remember a thing about it." However, this approach might lead to an alien invasion...but I say it's high time we took the care of this planet out of the hands of humans.