One of the fun things about worldbuilding is figuring out sundry things like how basic exchanges are reckoned!
I thought I had it figured out in "The Goldenlea," but I think I'm revising the old system I had in those books in favor of one that has a bit more basis in history for "The Linen butterfly."
Previously in "The Goldenlea," I had a three-tiered system of coinage. This was based on the Anglo-French model of the Livre, Sou, and Denier, and on the thought that it was mostly the same as pre-decimal UK coinage. But that was a mistake. In fact the Livre (Pound) was not a standard coin in its own right until much later.
In fact the Kings of France tried to standardize a Livre coin in the 13th and 14th centuries and failed because it was over- or undervalued. The Livre, for most of the medieval period, was a unit of account for large sums but not an actual coin of the realm. Most transactions were rendered in silver Deniers or the gold Sou, or Solidus.
I revised my Gold Sol/Silver Luna/ Copper Terra system. Instead transactions are rendered mainly in silver Terras and Gold Sola, and an additional unit of account, called a Handweight, has been added. Of course, larger sums can be rendered in marks for accounting purposes.