It's rude to show disappointment with gift.
Matt's mom bought him an expensive gift that he did not want.
Matt wants to return the gift and get something else.
Matt thanks his mom but later returns the gift to the store and receives store credit which he uses to buy something else.
Matt expresses how he feels about the gift and asks for the receipt.
Matt's mom is happy because she thinks he likes the gift.
Matt's mom thinks that he is being ungrateful about the gift.