My son is in second grade and him and a group of boys are always trading Pokemon cards, which is no big deal as they do it during recess and free time. Friday, my son came home with a Pokemon book. Nothing fancy or expensive, just a paperback that has about 30 pages. His friend offered him the book in exchange for 16 Pokemon cards that he wanted. The exchange was made and both boys were happy. This morning, my son was upset because the little boy told him he had to give the book back because he wanted to give it to someone else, but he's not wanting to give back all the cards. I don't think this is fair to my son, and he knows it's not fair either. I told him to tell his friend that there will be no more exchanges unless he gives back all 16 cards that my son had originally given him, as that is only fair. I know it might seem "petty", but if you return an item to the store, you get a full refund.

Was I wrong?