Agree to pay for chronic medication then decline payment

I duly followed the Discovery procedure to request an advance supply of chronic medication for an upcoming long-term international trip. The request was approved and Discovery agreed to make payment for the advance supply. When I went to the pharmacy to collect the medication, Discovery rejected payment on the basis that I exceed the monthly supply. When I contacted Discovery, the call centre agent said that I must pay in cash and claim it back from chronic care and that I should not worry. Three months later and I am still battling to get reimbursed. Now the excuse is that although the approval was granted in November 2017, the date of collection cannot be before 12 December 2017. I collected the medicine on 6 December 2017. I don't see what difference does it make when the medicine is collected. The point is that collection was made AFTER approval was granted. This is a feeble excuse not to honour the initial commitment.