No refund

I bought an X-Box game but then made a mistake took a game for XBox one instead of Xbox 360,When I get home my Boys open the disc did not notice was for Xbox one they realised it was incorrect. I took it back to Tb games Clearwater, I spoke to the manager who said they cannot refund as it was opened they I can trade it in for R100 or they will get the authorisation from their head office .I opted for authorisation from head office,I went back to the shop the next Saturday the Manager said NO the Head Office dicline the refund,I said to the Manager it is so unfair because I never signed anywhere to agree with their T&C.Can you assist me get back my R300 because I cannot use this game,I mean its so rude that I can trade it in for R100 while I've paid full R300