The only requirement is that a tribe decide to accept the in-game money as a currency. If (for exampole) the Shinra said we'll accept 40 dollars for a metal decoration, and we'll pay 20 dollars for 50 nails......you have a monetary system. People would know then if they want to buy 2 metal decorations, they'll need to sell the Shinra 200 nails, or 80 dollars. Not sure how heavy 200 nails would be to carry, but I'm sure 80 dollars would be considerably lighter. Which is why IRL, currency was developed to begin with.
For the currency to have any value to Shinra, they'd need to be able to use it to trade with other tribes though, so idealy, several tribe would start using money as an allowed currency exchange. Perhaps tribes could form some sort of alliance of tribes where they could agree and set a value on certain items of common exchange....or even without the alliance...tribes could determine the value of items to them. Labour could be included in the prices of things...like delivering sand to mountain tribes would be a trade some enterprising individual or small tribe could enguage in.
Sure, people can scavange for money, but it'd be like considering it a job...without a set sallary lol. Because everything will be able to be scavaged, for the most part...in game.
anyway, just my two inflated cents