This can work until you mint more coins than McDonalds can make Big Macs. Actually, that's not quite true, because people will put these coins into savings, and the system will persist until too many people decide to cash in that savings. Actually, that's not true either, because with these coins as actual currency the trading volume itself will absorb a lot of those coins. People will need to keep those coins in their pockets in order to pay for things. Banks will let you deposit the coins into savings accounts, and then lend most of them out to other people, who will use them to pay for things whose sellers will then deposit those coins in their own savings accounts, resulting in fractional reserve expansion. Credit card companies will issue cards denominated in these coins and let people rack up debt as well. Eventually, of course, everything will come crashing down, McDonalds executives will evacuate to their private islands while leaving their franchisees to be tarred, feathered and strung up.