Jack is a deathlands term for money, much like the word molla, or cash. Often a baron will issue jack in his own image to control trade, and to make the collection of taxes easier. The larger villes in more peaceful areas often have a common currency or jack. Usually villes will accept jak exchanged from another ville, however it will be at a rate favourable to that ville.
The old American greenback ceased to exist after the last weapons detonated. It is quite possible that in some area's local barons still accept it as currency, but for the most part, every ville or region produces it's own currency, or Jack as it has become to be known as. The currency comes in many forms, everything from home printed paper to minted coins. Most communities will only accept local or regional Jack, but many will buy it at a higher rate, say 3 foreign Jack for 2 local Jack. It is very rare for any community to accept foreign Jack at a one for one basis.
Still, the most common method of exchanging goods and services is done by trade or barter. If you want a meal, a bath and a place to sleep, it could simply cost you a few rounds of ammunition, or if your a female, a night of sex. That's the way it goes in the Deathlands.