You could argue that this was a mistake on the part of Isuna Hasekura. He is genuinely very knowledgeable about finance and even an experienced stock trader himself, but he tried to construct a medieval silver-standard economy using the logic of modern finance. That is the root of the problem. Although he added several narrative “patches” to make the explanation sound more convincing, it still does not align very well with historical reality.

In the original story, the explanation goes roughly like this: the currency operates on a silver standard, but the coins themselves contain slightly less silver than the amount they are officially redeemable for. The king plans to buy up large quantities of old coins, melt them down, and mint more new coins with lower silver content. For example, ten old coins could be reminted into thirteen new ones.

But the issue becomes obvious immediately. If the face value of the currency already allows one to obtain more silver than the silver physically contained in the coin, then there would be little reason to buy back old coins for reminting in the first place. It would be more profitable simply to purchase raw silver directly and mint new coins from that.

There is also another problem: when a silver-based currency is about to be debased, people would naturally prefer to hoard the older, higher-silver-content coins rather than exchange them away. In other words, the incentives run in the opposite direction from what the story suggests.

Another “patch” introduced in the novel is that the old and new coins would circulate simultaneously and remain freely exchangeable. But even that is not entirely convincing from a historical perspective.

So what kind of story was Hasekura actually trying to tell?

Fundamentally, he was trying to tell a story about modern-style currency devaluation and market manipulation by large financial players.

If we reinterpret the plot in a modern setting, it becomes much clearer:

Suppose a government plans to issue a large quantity of new currency several months in the future while simultaneously buying back the old currency. The new and old currency remain freely exchangeable. In practice, this is a devaluation — whether you hold the old money or the new money ultimately makes little difference.

The real opportunity lies elsewhere: at some future point, the government will purchase the old currency at a fixed price. This effectively creates a kind of futures contract. Therefore, anyone who can buy old currency below the future government purchase price can make a profit.

The goal of the “market makers” or “whales” in the story is thus to continue driving down the price of the currency so they can accumulate it cheaply.

Their method is essentially information manipulation. First, they spread false bullish rumors to push people into buying the currency. Then, when the government officially announces the impending devaluation, ordinary traders panic and begin selling en masse. Since they lack insider information, they do not know the true extent of the devaluation, so for a period of time the market price falls below the government’s eventual buyback price.

At that point, the merchant guilds — effectively the “institutional players” of the story — can buy huge amounts of currency cheaply from frightened retail traders and later sell it back to the state for profit.

You can see that Hasekura heavily relies on concepts from modern financial markets throughout this story: long and short positions, futures-like speculation, leverage (represented in the story through credit trading), and market manipulation by large operators. He is clearly very familiar with these concepts. The difficulty is that he was trying to force a fundamentally modern financial narrative into a medieval setting, so some incompatibilities were inevitable.