An invoice can be found at Teran Hall in Old Mournhold. It reveals the origin of a hole in the ground that is covered by a rug, causing the Nerevarine to fall to the lower level if stepped on. The invoice is on a shelf in the eastern area of Teran, near the Gatekeeper's chest.
Finished repairing the floor today. Please pay what you owe. Remember, I can let the goblin loose just as easily as I trapped him.