Central banks just print more money as needed to provide the amount needed to handle transactions. Or if they don't (or there is no central bank, if you're on

Central banks just print more money as needed to provide the amount needed to handle transactions. Or if they don't (or there is no central bank, if you're on a commodity gold standard), there is deflation, so the same amount of money as before can handle these transactions. Putting money under your mattress is a gift to society at large, as it means that you don't consume resources that you would have if you spent it.