"Sums are borrowed and lent between certain dealers in jewels, money changers, bankers, and the like. They have devised a means to transfer large amounts by means of written notes. Naturally, these notes are carefully done, and heavily magicked, but once executed are as good as gold!"Historically, banknotes, or something akin to them, were invented in China during the Tang dynasty (7th century CE). They were used in Europe starting in the 13th century, and soon became widespread. The Knights Templar were known to use such instruments to help develop their trade and finance network. Initially in Europe they were made out to specific individuals (like a modern-day check), but soon became payable to whomever the bearer happened to be.
This amazed the young thief, for he had imagined that he knew just about all there was to know about wealth.Gellor then explained to him that such instruments had a way of taking on a sort of mystical value themselves, becoming as dear as-- or even dearer than-- the physical things they represented, be they previous metals, gems, silk, spices, or something else.
-Gary Gygax, Saga of Old City, pp. 238-239
While counterfeiting is "as old as money itself", it's interesting that notes in the Flanaess are protected against such by magical protections. Naturally, counterfeiters are rarely stymied by any protective measure for long, and the art of crafting money (and notes) becomes a constant war of wits between the issuers, who have an interest in maintaining the integrity of their currency, and counterfeiters, who see great profit to be made from their craft.
Such a magical protection would need to be rare enough that ordinary criminal elements wouldn't be able to duplicate or defeat it, and yet not so rare (or expensive) that it would be unavailable to the creators of the notes themselves. The spells used in their creation would be carefully guarded secrets, as is the mundane process for creating the notes in the first place. I envision something like the following...
Level 4 mage, savant spell (alteration)
Requires: incantation, gestures, bank note, powdered gem
Casting time: 2 minutes
This spell allows the caster to protect up to ten bank notes (none of which can be more than 1/2 square foot in size), and alerts others to the presence of counterfeit notes by the fact that they are not protected by the spell, which is kept as a closely-guarded secret by the merchant guilds and clans of the Flanaess. Once a stack of notes has been protected by the spell, the identifying mark of the originator of the note (not the spell-caster, but the mark of the person or group against whose store of precious metal the note's value is promised) will be seen to glow subtly and move slowly. The spell's effect is permanent, but the notes do not gain any bonus to their saving throw vs. destructive forces such as fire or acid. If a protection from fire and/or a protection from lightning spell is cast upon the notes prior to the trustworthy note spell being cast, the notes will be permanently imbued with the bonuses of those spells. If more than 50% of the note itself is destroyed (torn in half, dissolved in acid, eaten, etc.) the spell will immediately cease functioning. The spell requires a powdered gemstone of at least 50 g.p. value be sprinkled upon the notes, so it is rarely used on notes of small value.
Naturally, this spell has fallen into the hands of certain powerful thieves' guilds, but they are careful not to over-use the secret, lest the notes become too devalued and their own wealth become diminished thereby. Certain illusion-type spells and effects can be used to duplicate the effect of this spell, and for this reason dispel illusion is often used by merchants when dealing with great sums (it should be noted that such is standard procedure when dealing with actual transfers of precious metals as well, as it is not unknown for a hoard of copper coins to be turned into platinum by the illusionist's art; such activities invite the death penalty in most civilized lands). Notes without the spell are perfectly legal and accepted in most places, but lack the assurance of trust that the spell's presence imbues upon them.
* This spell description is hereby designated as Open Game Content under the terms of the Open Gaming License.
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