Could make it so only the Banker could do it, Tom. Gives them something to actually do with money, limits who can do it, gives enemies a known target for warfare, etc.
What you COULD possibly do to limit unbalancing issues is give nobles the option to trade their bonds with the Banker (who would be the kingdom's official issuer of such things anyway) for gold. However, the Banker would need to have that gold in his own inventory at the time. Nobles give the Banker their bonds, the Banker gives the nobles his own gold as payment, much like the bank element as-is.
Gives infiltrators a target to assassinate/disable to hinder enemy campaigns as well as realm functions and gives enemy troops a target to arrest so as to take his stores of gold.
