What does this joke mean?

Because 2900 yuan is an integer, it is twenty-nine hundred-dollar bills. The maximum face value of RMB is 100 yuan, and rounding is meaningless.

I should be given 2900 yuan. If I give the cashier another hundred dollars, the cashier has to give me back the hundred dollars I gave him, so that's 30 hundred dollars, which is 3000 dollars, so I gave him one hundred dollars, and then he gave it back to me, which is redundant.

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Like this joke:

Once I went to the supermarket to queue up after shopping. When the person in front paid the money, the cashier said, "Your 100 yuan is fake." The man was surprised and said, "Impossible! This is what KFC just found for me! "