If there's a group out there that people trust to back a currency, then you can skip past the currency entirely. The central entity can issue trade credit in return for other goods or can convert goods into credit, thus guaranteeing them against theft, for a cut of the value. They keep a record of the goods on account and keep various amounts of trade goods in villages around the map. So instead of having to cross the world to trade a chest of E5 metal for a chest of A1 bulbs, both parties contact their closest central entity representative and make the trade with them. The bank gets a cut, the bank's clients get a quicker and broader market for quality-free goods like bulbs, low q metal, and robe grade silk.
So you don't need hearth dollars. If you trust someone to honor their debts and back their currency throughout the world, just use that trusted group as trade agents and you'll get much better trade improvements for the same amount of trouble. In time the bank's trading practices and exchange rates might give rise to a basic currency, but that's just a useful side effect.