If your time line is correct in the above your problem was that you gave her the 40 AFTER she'd wrapped it when she obviously thought you'd haggled down to 20 and was thinking 'well fuck him then" so gave you the one with the hole in it - alternatively we're both probably giving her too much credit as being a nice person who plays by the rules of decency as you were of course a tourist, which generally in any country means it's quite accept no actively encouraged to rip you off as they'll never see you again. I hope at the very least you can end your story with the line "but fuck her as the 40 I gave here were all fake notes anyway and then at least the balance between seller rip off merchant and fuck over the seller tourist will have been resumed and the universe and carry on as before ! :-)

Oh and yes, you're absolutely right re the 500 for the watch, I've been told that before too by the boys, hence my comment about me knowing I was done over too.