I think this is the right answer to the question.

Lets imagine it is not a fake note

Alice came to the John and buy the gift cost of $7. Alice pays $10.

John does not have change, he asks his friend Tom for change. Tom gives him the required change; and John gives Alice the gift and $3 in change.

Here you have to know:

Alice Has benefited $10 ($7 gift + $3 in change)

NOTE: If you think Alice benefited $17,or $20...  ($10 for counterfeit + $7 gift +?) is wrong.  Because Alice has only a gift plus $3 in change which is equal to the counterfeit note $10.  

John has his gift cost $7  (Tom has taken a fake note and paid for Alice gift as in change)

Tom has a note of  $10

After realizing that the note is counterfeit Tom has lost $10.

If Tom demands John then he has pay Tom $10

So, totally John is losing $10 ($7 for gift + $3 given to Alice in change  )

False Thinking: Most of the people think that the product has a values so, we have include that in the loss. But we have to realize that the product value is already add in the $10 ($7 product value + 3 change).

I obey with the answer $ 20

What ever the amount and materiel received by the Alice is the loss to the John

Alice gift cost  = $ 7

Alice payed bill = $ 10 ($7 for gift + $ 3 in change)

And,

Due to the counterfeit John has to pay the $10 to the Tom, but the John already has $ 7 (remaining money in Tom change ), So John has to add extra 3 rupees.

John Shopkeeper loss = Alice gift cost $ 7 + Alice payed money $ 10 + The amount going to pay due to counterfeit  to Tom is 3 $

John Loss = $ 20  

$10.

This is the amount Alice gained ($7 in good and $3 in cash) and the amount John lost.

The $3 John borrowed from Tom would be given back to him. So, that is a wash.