Refusing to refund or exchange a voucher

On the 03/02/2015 I went into spar Menlo park to buy telkom mobile airtime.i asked the cashier for telkom mobile airtime and she made a honest mistake and gave me telkom land line airtime. I didn't't realise at the time and after many attempts of trying to recharge I realised that she had made a mistake so I went into the spar and explained to the manager what had happened! After 20 minutes waiting he came out and said that they refused to exchange or refund MY money. I demanded to speak to the owner and was told he was not in so I left my contact details so the manager could phone me when he was in. After 2 days with no response or call I went into spar and asked why he Hadn't called? I was give some excuse and he was still not willing to help me I asked to see the owner again and was told he refused to see me! I find it pathetic the way I was treated and told I couldn't see him! I am the consumer here and it was the cashiers honest mistake! I didn't't want to go this route but had no other choice! I live with 5 people and we spend almost R10 000 a month there and get treated in this manner I ithink it's unacceptable