That is, if you've let out a road contract, and then say, "By the way, 20% of your pay will be in North Dakota vouchers", that would not fly. If you advertised in advance that those bidding to be paid in a combination of US$ and NDV will have the NDV portion of the bid weighed 2:1, and the bid was to be paid 20% in NDV, then that's just the terms of the voluntary contract between two parties.

I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.