I agree completely, all money has to go in the register....
I would think she put the entire $20 in, and had no way of knowing if the purchase was only $15 but, you don't get to take the $5.00 out and put in your pocket.
It still belongs to the store.
Besides, he wasn't there anyway !!
So then wouldn't that make it less likely for her to make up the story about seeing JLY that morning then? If she pocketed the 5 (not 15), then she would not have told such a story about him paying for 20 and only pumping 15.