How smart are you? What is the correct answer, page-14

Then there is GST as well. Of the $70 given back to the owner, $6.36 was given to the government.

So loss to retailer is $100 swiped from the till.
Gain from the sale is $70 (regardless of the change) minus cost price minus GST and any minus indirect cost like staff time to put it on the shelf and ring it through the till etc.

I would say the loss was $100, the rest is simply business as usual. Most likely someone else would have bought it anyway.