This was part of a weird dream I had but I've a vague recollection that it's based on a book that I read a long time ago, in my youth; a character had one of those change-makers that bus conductors used to wear with 'pods' of some description in each column. They then click for change that dispenses the pod into their hands which they then cast. My feeling is that the story was similar to the Spellsinger series, where somebody from our world crosses over to a magical realm and I probably would have read about the same period, mid to late eighties.
The laws of the five magics were being set aside. First to go were those of the high art of sorcery. Then true magic, upon which the commerce of all Arcadia depended, had been voided. And now even thaumaturgy, the engineering knowledge of the world, was under attack.
Jemidon had traced the trouble to Melizar, the strange cold being who had seemingly appeared from nowhere. Somehow, Melizar could alter or negate the laws that had always existed by using a mysterious metamagic - something which only he understood.
If the world was to be saved from Melizar, it was up to Jemidon to save it. But what could he do? He was only an outcast, unable to perform even the simplest ritual without ruining everything.
The coin changer is mentioned in this Reddit thread:
There's a coin changer in Secret of the Sixth Magic by Lyndon Hardy. It's a focus that allows it's user to disturb the laws of magic, which can then be replaced by different laws. It fits on a belt but it's not clear exactly how large it is, so I may need to find a hardback of the book.
Here's a few excerpts from the book:
In frustration, he rubbed the worn coin about his neck. There was nothing here that told him anything more than he already knew. Somehow, with greater ease than the simplest glamour, Melizar had changed the laws, replacing the substitute magic with yet another, Jemidon gripped the broken sword tighter, twisting its strange, unbalanced feeling back and forth with his wrist. Perhaps later, in the light of day, there might be something else that he could not see now. Yes, that was it—take an example of each form of magic and study the connection at a better time. He placed the sword hilt where he could easily find it again and then scooped up a handful of dominoes that lay next to the guard. He looked around for some example of traditional magic and saw Benedict's coinchanger reflecting the torchlight from a few feet away.
Jemidon stooped and pried loose the divulgenl's stiffening fingers from the device, which was still strapped to his waist. He cut it free and experimentally tripped one of the levers. A pile of worthless tokens fell into his palm and bounded onto the cavern floor.
There was no need to restore the art of sorcery, no reason to rescue a slave girl to prove that it could be done.
Or was there? Jemidon looked down at Benedict's changer. He pressed a lever, and a shower of brass and silver spilled into his hand. A single gold brandel gleamed on top of the pile. He picked it up and compared the sharp contours of the embossing with the dull indistinctness of the coin about his neck.
When the captain had gone, Jemidon turned his attention back to the coin changer and sighed. There was nothing else for him to do but wait. "If I start with three silvers before the galleons," he muttered, "then the first brandel will fall into the third column. That means that a dranbot must be next to deposit into the fifth."
While the masters exercised their skills, Jemidon emptied the coins from the changer into his palm. Quickly he sorted through the collection and reinserted them in the slit in top. He held his breath as he fingered his old worn brandel last and saw it slip away. Working the five levers one by one, he emptied the sorted coins back into his hand.