It's actually pretty cool. So for starters there's disambiguation since all serials are unique, you won't have carriers knowingly delivering to the wrong drop a

It's actually pretty cool. So for starters there's disambiguation since all serials are unique, you won't have carriers knowingly delivering to the wrong drop assuming the drop keeps possession of the bill since it's globally unique. You have some plausible deniability if found, "it's just money in my wallet, picked up at mcdonalds" and it's easy to hide/dispose of. If you're really smart you could also list a handful of possible serials for extra plausible deniability and hold/disperse the others such that the mechanism is still very likely to work due to the difficulty of finding a particular bill in circulation. Even when caught you may have some additional protection when the goods being immediately exchanged are otherwise legal, legal in the absence of money changing hands or difficult to establish in the absence of money or another component. Assuming the actual transaction is reasonably atomic, which it should be unless there's a fight or something, the the carrier can always prove that they delivered to the correct drop if they have a physical copy of the agreed upon serial while the drop can prove that they didn't receive a delivery if they can reproduce an undamaged copy of the whole bill. The ripping of the bill itself could plausibly serve as a step in a multi step process where the carrier may demand that the bill be ripped after the serial and goods are confirmed but before not yet exchanged so that the drop can't claim that a delivery was not at least attempted on the part of the courier. We could also do it with two bills, one from each party such that each party can have some proof that a transaction was or wasn't completed and with the right party in the case of a double exchange. The upfront cost of forging passable $1 bills is too high to be economical to pull off just for these low volume transactions, especially after the logistical cost of injecting the bills or otherwise tricking someone to using a fake as well as keeping a copy on hand but even if you achieved that. You'd also have to contend with the fed's coming after you for forging currency if the bills you distributed ever got into public circulation in any significant quantity so it should be pretty secure as long as they're sourced from a fairly high volume location like mcdonalds or something. The entropy might be somewhat useful too in bookkeeping like maybe using only bills ending with a 0 are for small cash transactions, 1 for large cash transactions, 2 is for drugs A, 3 for drug B etc... Entropy might be important but there are much easier sources.