mvpel mentioned a guy that paid his employees with uncirculated coins using their face value to underrepresent how much he was paying them and reduce how much taxes had to be paid. The IRS is apparently quite unhappy with him.
LOL! They might be unhappy with him, but short of wage laws I really don't think they can do a thing. There is a possibility that one will find coins worth more than their face value in circulation, and they are legal tender.
Now, if the employee were to sell the coins, then the IRS might have a way to be involved.
Were I the employee, I would find places to spend money where there were deep price discounts for those who made payment in gold and silver specie, and avoid the tax trap.
Somewhere along the line, though, they will try to tag someone for the difference when they sell the coin for Federal Reserve currency.