If none of the suggestions work, then buyer should open a not as described case and you will have to provide a return label, and on return refund. You can at this stage report to ebay, but do not expect any miracles, as their standard attitude is they did not see what was sent or received.

You may not think this is right, but if you do not go along with the system, buyer can escalate to ebay, who will refund at your expense, and you get a defect.

What exactly is buyer claiming to have received?