The buyer cannot use RTS if the parcel has been opened.  It is meant for parcels that cannot be delivered for some reason....not for free postage because the buyer does not like it.

If the return is for a genuine reason and has been accepted by the seller then the seller should send a return postage label.

The buyer has no proof of sending it back to you....it is their word against yours whether it ever arrives.  Do not refund until you get it back.  I suspect you never will receive it.