Merchants and monarchs

Local currencies, based entirely on the amount of produce in storage, devalue rapidly over time –  a sort of built-in inflation. That built-in devaluation induces people to spend the money by investing it in productive projects. Such local currency money is primarily employed as a medium of exchange. At the same time, central money is based on specie and used by international traders who could not typically gain access to the local stores of produce. Such central money is used primarily as a store of value. 

For several centuries before the 14th century’s Dark Ages, during the Age of Cathedrals (from the 10th to the 13th centuries), the standard of living for much of Europe was higher than it had ever been, present conditions included. 

It might well be the Renaissance is what kicked off the alliance between the merchants and the monarchs, an alliance that impoverished and weakened Europe so that it ultimately succumbed to the Black Death and the onset of the truly Dark Ages. 

The Biblical prohibition against the collecting of interest and against the dunning of loans beyond the shemitta year in effect stops the central currency from taking hold within the Land of Israel. That these prohibitions explicitly do not apply to trade with other peoples merely means the international currencies were of a different ilk from local currencies, and needed to be governed by different considerations.