I've alerted the Art Show department head to this thread, so hopefully she'll chime in soon...
My understanding is that when an agent is involved, the proceeds from art sales are paid to the agent at the convention. If this is indeed the case here, then your agent should have picked up the money at NDK, at which point it would be between you and your agent. I'm not sure what's supposed to happen if your agent failed to pick up the money at the convention, however.