"Ah thanks for taking the overdue taxes from the goblins. 100 gps and like agreed you will receive half of it."
[party turns to leave]
"Mhm... wait a minute. Are you registred taxpayers somewhere? No? Oh by the word of my lord you're now citizens of his realm and therefore have to pay taxes."
"Now all your magic items and the gold in your pockets sums up to roundabout 150,000 gps and a tenth will go to your new rightful ruler the king"
[Guards surround the party]
"Do you prefer to pay in gold or platinum?"
Bonuspoints if it is a good kingdom and a paladin (or something lawfull) in the party.