Suppose you are the 1st shopkeeper and I'm the 2nd
Its morning time and you have no single penny in your cash
Girl give you note of 1000, you took change from me and returned her 800
While returning her 800,
you kept 200 in your pocket on account of price of those goodies
Matlab yeh hoowa aap ne 200 ka shampoo becha aur 200 wasool paey
SO FAR it was a very much normal sale. No body knows about fake note.
No loss at all so far. Just a routine sale of 200 and you have that 200 in your pocket as real cash.
I disclosed in the evening that the said note of 1000 was fake.
You very kindly changed that fake note with a real one. This is the loss.
When you replace a fake note of 1000 with a real one, you face a loss of 1K. Thats it.
Its not simple but very simple. You just need to think the other way.
You can perform it practically as well to reach a correct answer.