At the time of Marco Polo's journey paper currency usage in China was heard of but not being used by Europeans. For short periods of time, often when losing a war, rulers would issue paper money in place of the metal coins. This happened periodically with no lasting circulating paper currency. Two notable examples are France and the United States however Sweden also experimented with paper currency. France, led by John Law and the Banque Generale issued paper money. However in only four years the bank, renamed to Banque Royale together with the Mississippi Company collapsed after a pyramid scheme. The United States in 1777 issued $13 million worth and continued producing more until 1780 when the printing completely stopped.

The exchange rates of various countries coins were easily judged. the coins were made out of the precious metal, the value was intrinsic. The copper, silver and gold respectively were worth if not all their weight an acceptable amount at least.

If you would like further reading I would suggest Jack Weatherford's History of Money