You could generate a key that way, but then we'd expect pre-distribution of each side. That would help confirm identities to both sides - if the courier and drop don't know each other, they just match the bill halves before dealing. As I understand it though, they're tearing on site.
So I think they're mainly trying to prevent the drop from stealing the money (claiming the courier never showed), and prevent the courier from stealing the money (claiming the drop has it). If C steals, D can now say, "I still have the Boss's whole dollar, why didn't C come take it?" If D steals, C can say "I have part of the dollar, proving delivery."
The boss is also trying to simplify this protocol, make sure low level drug offenders don't have to do a complex ledger entry during the commission of a major crime.
The tear might be purely symbolic, to make it look more symmetric to both parties, who may or may not have read Schneier.
Or... the tear could be preventing the D from ambushing C during the exit. Once the bill is torn, it would be hard to claim the courier never showed, even if they could get both parts of it back.
Sorry for only now replying to this, but are you saying basically the tear itself isn't actually that valuable in this process? That proving that your half matches up with the other half of the bill isn't important at all?
I see what you're saying otherwise, and it's not like I can really think of a specific situation where re-joining the dollar would be important, but I am curious if you've got any thoughts on how that might be useful, if at all.
