There’s no “things on his card” except his triggered ability. He attacks and trades with a scavenger. The scavenger and the taxman die simultaneously (state change action: units die when they take lethal damage), so two triggered actions are added to the queue: “opponent gains a gold” and “taxman steals a gold from an opponent.” As active player you decide the order of resolution of actions that happen simultaneously, and get to choose to resolve them in the order most favourable to you.