Lets think about this... You didnt mention a store so i will assume this is a chain store. Chain stores generally operate on small margins of profit of 1%.
Of the original $100 that was stolen only 1% of that was profit ($1) and the other 99% ($99) would have been used to repay the inherent costs. The stores original loss would have only been $99.
When the theif returns and spends $70. The stores profit is $0.70 and the inherit costs are $67.30. Even though the money was stolen from the store, it still turns a $0.70 profit from the sale.
We subtract the profit from from the second sale ($0.70) from the original inherent cost from the first ($99) and the actual loss for the store is...
98.30
Thats a smart cat at the register trying to minimize losses .