Exchange

On the 5th of December was my sons party I got gifts and some of the gifts are useless as my son has both of the same items or something similar to the other, so now I'm stuck with a gift that the store won't take coz there's no till slip I'm not asking for cash but to take something else of,the same value can they not look their records to see of this was bought there all the other shops dent have a problem the cashier says he knows the policy when I asked what it says he tells me he thinks. Why can't I exchange this gift