Did you know that your money is yours until you willingly give it away to other people? Charities can’t force you to donate, restaurants can’t force you to tip (unless they have an imposed X% markup on their prices), and any money that falls from your hand onto the ground is still absolutely yours.
This girl, for some strange reason, felt that finding a coin on the ground, which she clearly saw fall out of the OP’s hand, was up for grabs. So, she bent over and picked it up while the OP was still around. Strange behavior, indeed. Guess her mama didn’t teach her about not taking what doesn’t belong to her.