Refuse to claim voucher

I was due for a service. I claimed a voucher R500 for service operation more than R2000. The service manager Charne refuse to give the R500 discount and only give 10% discount. I phoned Head office and they said she must give it. When headiffice phone Charne she said no I must read term and conditions. We read it and it is not stated that after claiming the R500 the balance must be R2000. I am really unhappy. Can you okease solve this problem. Reanult Port Elizabeth