Answer:
He lost $5.
Explanation:
When the customer gives him the $20, the salesman does not gain any money because it is fake.
Then, he gets real money summing up to $20. So he gains that, but then gives $5 away. Now he has $15.
Then, the lady tells him the $20 bill is fake, so he must give the lady $20. So, 15 - 20 = -5. He lost $5.