Interesting. A search of News Articles brings up only four (4) instances of the phrase, "negotiable American currency."
1927 - NY Daily News, in an article about the purse of the upcoming Dempsey vs. Tunney rematch
1958 - The Montgomery Advisor (Ala.), Politics section, in a nameless editorial rant about money in baseball
1970 - (*See Attachment 1)
2011 - In a Canadian article about the Citizen Sleuths.
The 1970 date stuck out to me, and I read that one first. Then it mentioned counterfeit money, which peaked my interested even more...
And then I removed the word "American" and searched again, and was wildly underwhelmed by the over 3,000 instances of that phrase being written in newsprint, from all regions and most states of the country as well as Australia, Canada and the U.K. Everything from police blotters to financial news to freaking horoscopes has the term "negotiable currency" in it. One instance was a lawsuit with an order to pay the plaintiff in "negotiable currency of the United States."
Verdict: the term is not altogether irrelevant, but does not establish likelihood of being Canadian or otherwise foreign. At best it shows that Cooper was better at communicating than your average Warner Brothers cartoon mob goon.