This is a very simple but confusing puzzle.
A customer buys goods worth 200 rupees from a shop. The shopkeeper selling these goods makes zero profit from this purchase.
The lady gives him a 1000 rupee note.
The shopkeeper has no change, so he goes next door to another shopkeeper to get change for the 1000 rupee note. He keeps 200 for himself and returns 800 to the customer.
Later, the second shopkeeper from next door comes back with the 1000 rupee note with a stamp on it saying "counterfeit" and takes his money back.
How much loss does the first shopkeeper face?
The shopkeeper pays to the customer 800 + he returns 1000 to the other shopkeeper from whom he had borrowed. The answer is 1800.