Europe, as I mentioned, came rather late to the Middle Ages and for most of it was something of a hinterland. Still, the period began much as it did elsewhere, with the disappearance of coinage.
Money retreated into virtuality. Everyone continued to calculate costs in Roman currency, then, later, in Carolingian “imaginary money”—the purely conceptual system of pounds, shillings, and pence used across Western Europe to keep accounts well into the seventeenth century.
Local mints did gradually come back into operation, producing coins in an endless variety of weight, purity, and denominations. How these related to the pan-European system, though, was a matter of manipulation.
Kings regularly issued decrees revaluing their own coins in relation to the money of account, “crying up” the currency by, say, declaring that, henceforth, one of their ecus or escudos would no longer be worth 1/ 12 but now 1/ 8 of a shilling (thus effectively raising taxes) or “crying down” the value of their coins by doing the reverse (thus effectively reducing their debts).
The real gold or silver content of coins was endlessly readjusted, and currencies were frequently called in for re-minting.
Meanwhile, most everyday transactions dispensed with cash entirely, operating through tallies, tokens, ledgers, or transactions in kind.
As a result, when the Scholastics came to address economic questions in the thirteenth century, they quickly adopted Aristotle’s position that money was a mere social convention: that it was, basically, whatever human beings decided that it was.