where was salt used as commodity money

The term you are looking for is commodity money. Some examples of commodity money are gold and silver.

A commodity is a good that is worth money, there is no such thing as "commodity money". So if you have a good that was purchased from a vendor that is by definition a commodity, its value is whatever you paid for it, my suggestion is a mark up and that is its profit.

commodity money is a good that can be used as a medium of exchange or for some other purpose

What is the difference between money and commodity?

Commodity money is a sort of money that is considered as a present good. Whereas, fiat money is a future obligation as it is simply a promise to pay in the future. Payment is never made when it comes to fiat money, instead it is only discharged. But commodity money, on the other hand, completes the transaction.