"From behind" joke

A
woman was walking down the street when she was approached
by a man. The man said, "I must have you right
now! I'll drop $500 on the ground at your feet and in
the time it takes for you to pick it up I can have my
way with you from behind!"
The woman thought it over and told the man to wait
a minute. She called her girlfriend on her cell phone
and told her about the man's proposition. Her girlfriend
said "When he drops the $500 on the ground I'm
sure you can pick it up and run before he gets his
pants down. Call me back and tell me what happened."
An hour and a half later the lady called her girlfriend
back. "What happened?" the girlfriend asked.
The lady said "That jerk had $500 in quarters!"