Code for exchanger NPC:

/playsound mob.villager.haggle @initiator[hasitem={item=gold_ingot,quantity=..31}]
/title @initiator[hasitem={item=gold_ingot,quantity=..31}] actionbar You must bring more gold!
/playsound random.orb @initiator[hasitem={item=gold_ingot,quantity=32..}]
/title @initiator[hasitem={item=gold_ingot,quantity=32..}] actionbar Thankyou for using the exchange!
/give @initiator[hasitem={item=gold_ingot,quantity=32..}] iron_ingot 32
/clear @initiator[hasitem={item=gold_ingot,quantity=32..}] gold_ingot 0 32