say what?!

So, I called the electricty company to give them the receipt number from paying my bill this morning and got a huge surprise...I guess the lady who took my payment entered it wrong...It shows that I paid $600 instead of only $200, and that the electric company owes me over $100 for over paying. The lady in the billing department was very nice and said that unless the payment center notices their mistake the balance stays the way it is...Now, being the honest person that I am, I would USALLY go back over there and let them know about the mistake. BUT!...the lady that took my payment was very rude about the fact that they don't take debit cards, so I had to go find an ATM and come back with the cash to make the payment...and since they only deal in cash and my receipt shows that I paid only $200 I don't think I'm going to correct them...If they find out and fix the error thats fine, but if not oh well and i'm finally ahead for once...