Producing enough to have surpluses is not really sustainable. I expect the usual pattern if people do survive post crash is that the population group will again to follow the pattern of rising production and consumption, until overshoot and collapse is reached. Money and trading are part of this pattern of getting to overshoot and collapse.
If a population is to be sustainable, it must really operate at a very low level. I expect that having language is already too advanced. Having tokens for money makes the population even less sustainable. Being able to train dogs to help in finding food is another unsustainable type of activity. We gain too many advantages over other animals, leading to our overshoot and collapse problem. - Gail Tverberg