Acdc express Worcester complaint

Went to buy a cable at Acdc Worcester , when I was done with the purchase, I went outside to answer the call from the person who sent me, and he informed me that it was the wrong cable size he gave, went back inside the shop and informed the person helping me that it was the wrong size and if I could return it and get my desired cable. He said that they can't take back the cable and didnt give a reason for why. My reasoning is that why can't they just take back the cable and give me shop credit in return , and deduct the amount from the other cable as it is more expensive than the cable I bought.

Details : ACDC EXPRESS WORCESTER Contact number :*** Adress: Kwv, Corner Church and Smith Street, opposite, Worcester, 6850