The best currency is obviously tally sticks.

I joke - I haven’t used this in a game. It is interesting, though. From what I understand of history, the vast majority of people used a gift economy, barter, and what was essentially a credit system to get what they needed. The idea of the adventurers ‘buying’ their equipment on credit by owing the blacksmith a portion of loot later does raise some ideas.

Also tally sticks are where we get the phrase ‘stock holder’; the one who loaned the goods kept the larger end of the split tally, which was called the stock.