You go to the bank to trade a dirty dollar for a new , clean one , the cashier tell you the trade is not necesarry becuase currency is

Hey there!

The answer should be "stable in value", or something similar. 

The condition of a dollar or any other kind of currency doesn't really matter, unlike something like knick knacks or clothing, which usually drop in price as they get dirty or worn. Currency, however, will still have the same value, even with cosmetic damage. Therefore, it's unnecessary to get a new dollar bill for an old, dirty one, since the value is still the same. 

Hope this helped you out! :-)