Thank you both for the warm welcome. Georger, I wasn't familiar with Fitzgerald's work, although I certainly remember that the Unabomber manifesto was the subject of intense analysis. In fact, I had a highly plausible candidate at the time, based on the manifesto, before Kaczynski was apprehended. Needless to say, I am not an expert forensic linguist, and I was wrong. I have read Don Foster's "Author Unknown." I enjoyed it, though I think he actually has a bit of a tin ear, especially when it comes to Shakespeare, his specialty. I'm fascinated by idiolect, but have a high bar for being convinced of conclusions. Language is a slippery thing.
All that said...I understand that Cooper himself may not have said "negotiable American currency." I am too green, by far, in all things Cooper, to trace when it was attributed to him. Someone said it, though, and the question of whether it's common, or proper, American usage is an easier question. Which brings me to Robert99's questions. In brief, I searched the entire corpus of Google Books and the Internet Archive for the two phrases- "negotiable American currency" and "negotiable U.S. currency." There were not many hits, a few dozen in Google, and a lot fewer in the Internet Archive. There were many, many more hits for the phrase "negotiable currency." Too many to be useful. I skipped over the hits related to DB Cooper books. Of the remainder, I looked for quotes from a variety of types of writing- fiction, non-fiction, pre-1971 and post-1971, etc. I did enough quick research on authors to determine whether they were native speakers of English, raised in the US. My search was not exhaustive, or scientific. But in this case, I don't think it has to be.
My conclusion is that the phrases "negotiable American currency," and "negotiable U.S. currency" are not common, but are not super-rare, nor are they incorrect. A couple of the uses I found seem to be intended as mildly humorous- the writer deliberately using a "fancy" phrase, rather than saying something like "cold hard cash." Several of the uses are just literal and descriptive. The final example, from The Currency Dealer Newsletter, likely has a more technical financial meaning. The most straightforward definition I could find for negotiable instruments is the following from an online business dictionary: "Negotiable instruments are unconditional orders or promise to pay, and include checks, drafts, bearer bonds, some certificates of deposit, promissory notes, and bank notes (currency)."
IF Dan Cooper did use the phrase, it could indicate some background in banking or finance. On the other hand, he could have simply been being very precise, in the manner of an engineer or scientist. He could have meant "I don't want gold bars, Mexican pesos or untraceable bearer bonds. Nothing exotic-this isn't a movie. Just give me regular negotiable American currency."
P.S. As for the phrase "let's get this show on the road," this seems to be primarily an American saying, and very common, to the point of being a cliche, especially in the 1960s.