I still remember when I was a child, when my parents told me to go to the store to get coins to pay at the stationery store, my hazy memory still couldn't figure out why the coins I took out could be exchanged for the mechanical pencils I wanted. Tong Yan Tongyu's head, the immediate intuition is: "Why can't I take a used automatic pen and exchange it with the boss for a new one?"
From the perspective of the contract, if the store owner is willing to exchange your old automatic pen with a new one in his store, it is of course in line