1 day over exchange period and refusing credit

I wanted to buy my mom a jersey I asked the sales assistant should I buy the jersey and she doesn't like it would I have a problem exchanging it otherwise I will rather not take it and bring her with me they said no take it if she doesn't like it we will give u a credit note valid for 3 years I thought oh ok that's good enough. I went to the store 7 days later they were closing and I called I have phone records showing the call to ask if they could help me searwin said no he is busy closing. I went back the Monday as it was the only next opportunity I had to go back and they refused to credit me I then emailed head office coz it was 1 day and I was there 7 days ago as well and I told the assistant before purchasing it that it might need to be returned. Their reply was they would only exchange it within 2 days and refused to give a credit note. My mom is not well at the moment and I can't take her to choose something what if I take something and she doesn't like it again . I mean it's not like it's a week it's one day and I was there 7 days ago I called them to tell them I need to return it .