Blame this.
Mage Alicia Cogstalker’s Astounding Treasure Machine
The existence of this magically-engineered device is prima face evidence of the existence of dimensional travel: clearly an enchanter came into contact with an automated teller machine, and loved the concept on sight. Alas, the average late-medieval/early Renaissance culture has little hope of duplicating cathode-ray screens and transistors, to say nothing of hard-to-counterfeit paper currency. But one may, as they say, make do.
The Treasure Machine (unlike many magical artifacts, its creator is happy to have people use the ‘ATM’ acronym; that’s why she picked the name, after all) screams ‘of Gnomish manufacture’ in such worlds that have such things. It’s all brass and wood and clockwork, with a flat screen of glass where things can be written by the resident embodied spirits – NOT IMPS. THIS DEVICE HAS BEEN CLEARED BY EVERY LAWFUL GOOD ORGANIZATION IN THE CAMPAIGN WORLD – in much the same way that we use an Etch a Sketch, only from the inside. Most of the inside is actually a residence for those spirits, in fact; each ATM has about two or three hundred spirits living in it. If it wasn’t a gross violation of privacy, replacing one wall of the ATM with glass would show a busy community maintaining the device, going up and down the internal elevators (easily the most mechanically delicate part of the mechanism) to the display screen, relaxing after work, worshiping the permanent teleportation portal in the center…
Yes, that last part is how this entire thing works. ATMs only work with one bank, at present: you make a request for, say, ten gold coins from your account. The spirits toss the request into the portal: if the bank confirms that you have ten gold coins to spare, it throws them through the portal and then the attendants spirithandle them into the dispensing hopper. If you want to deposit ten gold coins, you put them into the receiving hopper with a note of which account they’re meant to go, the spirits get them through the portal, and you get a tentative receipt for delivery. The entire process is mildly cumbersome and not really designed for use by anybody except aristocrats, wealthy merchants, and adventurers… which is to say, the exact sorts of people who could effectively complain about this, so no worries, right?
Besides, it keeps a lot of embodied spirits usefully employed. And out of trouble. And disinclined to try the entire ‘imp’ or ‘gremlin’ lifestyles. So, hey: net gain to the community anyway.