Double billing

In March I took my car for a service at GM Honeydew, only recently when I took it again for service at GM Cresta did I notice that Honeydew double billed me on oil. I called Honeydew yesterday(10/11/15) and Caroline confirmed that they double billed me and I should give my banking details for a refund. I told her I'll come into the dealership as I needed more clarity on other items in invoice.<br> <br> I went to the dealership this morning(11/11/15) and to my surprise she tells me I cannot be refunded in cash, I can only be refunding through transferring funds into another dealership for my next service. But I have recently done my service, the next service will be next year. This to me shows that they're not even apologetic for double charging me. When they wanted payment, I made it immediately, why is theirs conditional?