My job was to promote something in a grocery store. I happened to cross paths with someone from the bakery and they asked if I wanted a pizza to draw interest in my permission. I said ok. After the pizza was made someone else from the store said I had to pay for the pizza. Legally speaking must I have paid for the pizza? Who would have the authority to decide if it's free or not? Must they have told me the cost prior to giving it to me (and it being consumed)? Conversely there's an expectation when getting something from a store that you need to pay for it. For example if you didn't see a price tag on something you probably wouldn't assume it's free.
A similar situation was my friend in school went on a trip with debate team. The club failed to get a discount and asked him to pay more than they originally charged. He refused. Was this legal?