Examining this from a cost-value reference, assuming the $100 counterfeit was originally in the customer's possession (knowingly or unknowingly.)
Initial values will be set to Zero.
Start - Shop A, Shop B, and Man at $0.
The man goes to purchase the item with a counterfeit $100. No value changes hands. The Item is held by Shop A still.
Shopkeeper A goes to Shopkeeper B for change. Thus getting $100 in legitimate bills, while passing the counterfeit off to the other shopkeeper.
Shopkeep Change Shop A is +$100. Shop B is -$100. Man is even.
Shopkeeper A returns to his store and gives the customer his item and $69 in change, a total value of $85. The man departs.
Purchase Completed Shop A at +$15. Shop B at -$100. Man at +$85.
Shop B discovers the counterfeit, and Shop A is forced to eat the cost.
Final Result Shop A loses $100, now at $85 loss.
Shop B is even, and, of course, the patron is the recipient of the $85 in value that he (knowingly or unknowingly) did not pay for legally.