Are you saying you're not allowing yourself to buy fabric even for WIPs? Wow. A bigger challenge than I'm setting myself. The way I'm doing it (which is what KiwiQuilter suggested initially, but of course we can all make our own rules!!!) is that fabric can be bought as needed to finish a project. I guess I'm just not a scrappy lover...
In any case I'd suggest that trading fabric would be ok because it doesn't cost me money and won't swell up my stash. But that's just me. You might have different reasons for doing this, so it's your rule to make.... :)