[Piskvorrr]

"Not for very long" - that's exactly the point of the currency-exchange scam:

- mark gets an offer of a great exchange rate from the con-man

- mark hands over real money

- gets a stack of something - the first few bills on the top and bottom are actual local currency, but most of it are one-sided counterfeits.

- con-man counts the money in a way that doesn't give this away - this obviously requires sleight of hand and insufficient lighting

- con-man disappears in crowd

- mark eventually realizes that most of the stack is worthless

The whole con takes a minute or two at most: that's not very long at all.

The critical section "mark no longer has original currency but hasn't yet realized he has been swindled" is as short as a few seconds; this has even been done with plain paper in some particularly audacious cases.