As with any form of currency in Fallout: New Vegas, $20 NCR is not affected by the Courier's Barter skill. The currency will always be valued at 8 caps, regardless of whether the Courier is buying or selling.
Behind the scenes
A cut version of $20 NCR exists in Fallout: New Vegas' resources. It is a non-currency item, meaning its price is affected by the Courier's Barter skill. It is otherwise identical to the standard bill.