Negotiable means to trade-up. It is illegal fiduciary responsibility to trade-down. We always trade up in our negotiations - the essence of win-win.
Therefore Fed notes are negotiable because there is a higher form of currency allegedly available to trade-up to - US notes. In form, since
US notes cannot be used as a reserve currency and there is a set amount of them in circulation (non-elastic) they are a higher form of currency. US notes are non-negotiable. The only thing you can trade-up for is energy - material goods and services.
Since January 21, 1971 FRNs function for US notes. Therefore due to Treasury policy, FRNs are actually performing as US notes too. So if you have endorsed private credit the FRNs in your pocket are FRNs but if you have not, then the FRNs in your pocket are US notes. By renaming them
US currency notes Congress has adjusted the face value to be the same.
In my opinion Congress has defrauded itself, if that is possible.
My suggestion, at least for clerk of court transactions - which means every bill in your wallet if you are a court of competent jurisdiction is to buy one of these stamps.