Do you think the shop clerk was trying to rip me off or does this sound like it was an honest mistake?

yesterday i went to the convenience store at a gas station. i bought a cup of coffee and a buttered roll. the total came to $3. I had 2 twenties and 2 dollar bills in my wallet, so i gave the clerk a $20. He gave me $2 back. I said to him “I gave you $20.” He said “no, you gave me $5.” I said “Oh, i thought i gave you $20.” He had the drawer open, and he looked at it, and he said “No, you gave me $20.” I said OK and started leaving. I was thinking maybe i was mistaken, what proof do i have, what could i do about it anyway, which is why i did not argue about it. As i got to the door of the store, he said “Oh, you did give me $20.” When I returned to the counter, he said “I looked in the drawer and saw I put the $20 in the $5 spot.” I thanked him several times for being honest, and he gave me $15, which was the proper change from my $20.

I asked a friend for his opinion. I was wondering if the guy tried to rip me off, or if it sounds like an honest mistake. My friend’s opinion was that the guy was trying to rip me off, but then since i “caught” him he decided to be honest about it. My friend admits he could be wrong, it could have been an honest mistake.

There is no proof of what the shop clerk’s true motivation was: honest mistake or trying to rip me off. I am just wondering what other Fluther members think it seems like.