Enforcing payment when account is in positive balance.

I have a positive balance on my Discovery Credit Card but I am however expected to still make a payment. When speaking to their agents they tell me that my account is in a positive due to a refund and that a refund doesn't count as a payment. How can I be expected to pay Discovery Bank if I do not owe them any money. In fact, my account is in a positive therefore they owe me money, or not? I was told that if I do not make the payment, it will reflect negatively on my credit score.