You cannot pay anything because it is already paid for, all you can do is exchange one for the other. The states convert your labor hours to debt entries so you can go exchange for credit entries.
You go exchange for goods and services the receipt is the credit, it is backed by substance.. think about it. A receipt then can becomes true lawful money and can be used in credit swapping.
Bankers and brokers call this a Credit Default swap, IRS also has a procedure called Notional Principal Contracts. Prepay then becomes a remarkable tool to learn new tricks with.