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The English word “money” as currently defined has nothing to do with Kesef, Zahav, etc. In fact, there is no word in the Hebrew language for “money”. The people of the time of the gemara (Jews and goyim) had no conception of what we in the 21st century refer to as “money”, and probably would think we must be totally insane to confuse what we call “money” with what they used for money (many conservative economists would probably agree with them).
The word modern Israeli word “kesef” actually refers to “silver”. Modern Ivrit has adapted it to refer to “money.” In modern English, money consists of entries in bank accounts, which exist in online databases (for the most part) and are typically exchanged via the internet. Some people still make limited use of paper certificates or coins made of worthless metal, but their use is being phased out. Kesef and Zahav are rarely used for commercial purposes on a daily basis (can you imagine offering a one-ounce coin to pay for pizza).
However since the modern economy totally runs on what in English is called “money”, which is a virtual unit of relative value with no intrinsic worth and certainly no fixed or stable worth – it is impossible to apply any psak halacha about what was used for “money” more than a few centuries ago to the modern economy.
Thus talking about Rambam and “Free market economy” makes no more sense than talking about Rambam and the telephone, or Rambam and the computer.