Cancel a purchase

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I placed a bid on an item and won.  The seller contacted me and said they were out of stock and it would be a long time before they could ship and would I like a refund.  I said yes.  Now I received a note to complete this cancellation transaction and seller says I ordred in error which is not the case.  How would you suggest I respond.  I do not want any negative on my end.  It was not my fault.