Currency exists because gold(or whatever is the standard of money) is hard to move around.
Credit/Bonds exist because Currency can be hard to move in large numbers(think 1 million gold coins).
But when times are unstable, laws are slow to update, and banks are unable to exert influence across multiple countries, trading bonds from one bank to another is not possible: only Currency is accepted.
Because of that, Currency itself is a very important commodity for cash transactions. Since monsters limit both the production and distribution of it, Currency gets additional value on top of what it represents simply because there's a high demand and not enough supply.
So Rblade was in some trouble due to lack of physical currency, which Fullgold had in surplus. That was a weakness which Fullgold exploited, baiting them with the solution to their problems in order to crush them.