Answer: The total loss incurred by the store owner is 100$.
At first look, the answer might seem to be 200$ or 130$, but that's not the correct one. The explanation is below:
The lady stole 100$ from the owner therefore loss incurred is 100$.
The lady went back to the store and:
Step 1 - The lady hands back the 100$ to the store owner. Loss incurred is back to 0$.
Step 2 - The store owner hands over merchandise worth 70$. Loss Incurred is now 70$.
Step 3 - The store owner hands over 30$ worth of change. Loss Incurred is (70 + 30)$ = 100$