Asking for multiple payments

I had ordered a water purifier from water science for COD. They called to say that cod is not possible and I had to pay. So I paid 1214 first. They said it is 1204 for online payment and asked me to send this amount again, which I did. They said they cannot refund the first payment due to some error on their end. I told to cancel and return my full amount for which they are asking me to send 2418 in a single transaction and only then they will refund, which I did not agree. Now my 2418/- is with them and they are refusing to deliver the item and refund the amount.