From what you write, you would owe the fee--you returned the dog within 24 hours, which would obligate you to pay the fee. The fact that someone forgot to charge you the fee on the spot doesn't invalidate their claim to it or your obligation to pay. Consider: suppose you worked for someone on a freelance basis, and did 9 hours of work at $30/hour. You forgot about one hour and billed for the first 8 only. Legally, if you then remember the hour and bill for it, your employer would have to pay--the fact that you didn't charge at the time does not mean you can't receive compensation. It's the same principal here--if you owe the money, you owe the money.