What is correct answer? 

Simple, $100.

Consider this: if you asked the question, but the person who bought the $70 item was a different person with a different $100 bill, then that would be quite obviously irrelevant.

So the question is, what difference does it make if the lady buys something or someone else does? And the answer is none, he sold an item and received money for it. That is independent of the earlier theft.

EDIT:

Here’s an even better answer: What does the question mean by “How much did the owner lose?” I would argue, that since the owner was just robbed, the “loss” is how much was stolen, not anything to do with profit and loss, or cost of goods sold or anything like that.

So at the end of the day, he counts up his money and finds that the till is short. How much money is missing? $100.