A question that forces you to think

Scenario: A lady comes to a shopkeeper to purchase a pen. After looking over many options, she decides to go with a pen which costs Rs.80/-. She gives a Rs100/- note to the shopkeeper. The shopkeeper doesn’t have Rs20/- change and hence approaches another shopkeeper with the Rs.100/- note and gets lower denominations of the same value and returns to his shop. He gives the lady customer her balance amount of Rs.20/- and she leaves the shop. A short while after she leaves, the other shopkeeper approaches him complaining that the Rs.100/- note he gave him was fake. After an argument, the shopkeeper finally takes back that Rs. 100/- fake note and returns the same amount of money to the other shopkeeper.

Question: Calculate the loss of the shopkeeper who sold the pen?