Hi,
I am not able to understand the following para from Manhattan SC (Chapter 5, page# 74)

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Upon leaving the register, the cashier handed the customer a receipt.

Here, the modifier "upon leaving the register" seems to modify "the cashier", although it should modify "the customer". In order to correct this, we must place the modifying phrase directly next to what it modifies.

Upon leaving the register, the customer received the receipt from the cashier.

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My questions are
1. Why is the first sentence wrong?? The sentence looks OK to me as does the 'corrected' sentence which appears later.
2. Why cant the "Upon...." modify cashier?? what is wrong with that??

Can someone pls explain?

thanks
-Vittal