Just as the title says. I could add the script to take the money to something outside when the player leaves the building, and I know how to do that, but I want the immediate removal of the "gold" item as soon as the player clicks that dialogue choice and finishes my quest.

Firstly: Create a properties, Choice at Type "Misc Object" and give the name of the properties. Then write this code.

Game.GetPlayer().RemoveItem(Reward,1000)

Reward-It's gold which i chosen in the properties
It's code remove 1000 golds from the player