The tally stick was typically broken in half and the splintered ends proved the pair were mates.  So one person was creditor and one was debtor (or it could prove a contract between them).  The rest of the stick showed how much (or details about the contract).  The creditor could then sell the "debt side" to someone else, and the debtor would then owe it to the new owner.  One could collect a bunch of them and say "most of these people will pay me the debt" and then go into debt himself with a second layer of tally sticks on the collateral of the batch of 1st sticks he holds (a derivative).   Even without this a bubble of loans could build up on tally sticks, and with too many of them floating around, but it would temporarily stimulate the economy by people buying things.  Merchants could raise their prices if they thought people were issuing each other too many tally sticks.  Holders of lots of tally sticks could sell them out of anticipation they would become worth less from too many of them.  So same thing as money.  ((  BTW   The Fed is buying toxic waste derivatives from banks and using them as collateral against the issuance of newly-created dollars, so QE is a derivative on top of derivatives.  The Fed buying worthless U.S. treasuries is also a derivative because the treasuries were initially issued in order to get money from someone but that money also had to be printed some time in the past, so they're printing money using the original printing as the collateral for the 2nd printing.  ))

Since bitcoin is not used for anything else, it's not a commodity.  It's an asset.  Assets that are not used to generate income like machines and are not a commodity being stored have prices that are determined by public perception.  Artwork is a good example that is like bitcoin.  House prices are halfway between perception and reality.  The creditor side of the tally stick is an asset and the debtor side is a liability.

Bitcoin is an asset that can function like a currency and a contract (using its other features).  The splintered ends of the tally are a key pair.  The debtor end would have be the public key and the creditor key the private key. The ledger of bitcoin is like the public debtor side and the private key is the creditor who paid something "into the system" to obtain the "debt instrument" private key.  

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So the bitcoin ledger is a debt "the public bitcoin system" owes to each "creditor" (bitcoin holders).  It's a pretty good analogy except the tally sticks were not limited in supply.