An Encounter With A Con Artist

I am still in the state of shock as I am typing this post. I had an encounter with a con artist last Saturday night. He (the con artist) was my second to the last customer before we closed. He came to our store, bought one item which was a dollar, and after I rung it, his due became $1.06 (including tax). He gave me a $20.06, gave back to him a $10, a $5 and four $1 (total of $19.00) He took the money by his left hand, and 5 seconds later, he move it to his right hand and gave it to me saying, "I'm suppose to get $19.00 back, this is only $9.00) he didn't even counted it. I knew right then that he had tricked me. I took the money, counted it with disbelief. "How did you do that?" is the question I want to poke at him, but didn't have the guts to do it. I knew I gave him the right chance because I only had 2 $10's and I gave to him one, but I can't prove it to him. "This man is a con artist," was the words that repeatedly ran on my mind while I was looking at him as if trying to dig any feeling of guilt in him, but of course, as a con artist, he just acted like he was innocent by talking too much about his whereabouts. The good thing is, I can still recognize his face, and whenever he shows up, I will know what to do then.
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According to Wikipedia, a con artist is a person who do some confidence trick or confidence game. Their business is to defraud individual/group from all walks of life by gaining their confidence, and then victimize them with their skills.
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I know we can't keep them from going around, victimizing people; and we can't avoid them either. I didn't say that I am ready to face them the next time they attack me, in fact, my prayer is that it won't happen again, but if it have to, my goal from now on is to be more bold and vocal in rebuking them, with prayer to God.