If they don't have dollars, they can pay each other in IOUs (which aren't divisible, but otherwise would work fine).
Affordability is about more than simply having some means to pay, though.
I often struggle to find a word that fits well here.
The dictionary defines afford to mean: 'to be able to do, manage, or bear without serious consequence or adverse effect'.
By that standard, some people who can't afford to go to college actually do. The means are available via loans, but there are serious consequences and they will be saddled with large amounts of debt. So what's the term for that?
I don't think 'afford' as a proxy for 'strictly possible' is very useful and I'd be glad to hear of an alternative.