Background
I purchased something from a local store. The salesperson made a mistake and quoted me a lower price than what should have been. I made a down payment since they didn't have what I want at that moment. A week later I paid the remaining and got the goods I asked for. Days later, the same salesperson called me (I left my number with the down payment), saying he under-quoted one of the few items. He asked me (with good attitude) to pay back the difference. I decided to be a good guy and deposited the difference to the store's bank account.
I though wonder if I am legally required to pay them back.
Question
In the case that a salesperson made a mistake and gave an underpriced quote to a customer, after the service had been served or the goods had been delivered, and the customer had paid in full, can the salesperson (or the store) legally demand the customer to pay back the difference?