The buyer will need to open a return case through Ebay.
You can provide them with a label if you want them to send what they did receive to you but, ultimately, you will have to refund them.
It is possible that parcel labels have been switched around.
If you don't believe the buyer is genuine and they are trying to scam you, come back for more advice.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
