You dont have choice i am afraid, as soon buyer will open claim, paypal will put your paypal account to minus.
I dont like statement from buyer: As I sent you the money before receiving the phone I would like the same curtesy from you.
I would contact buyer, telling her she can open claim trough paypal, only when phone received refund will be issued. Also warn her when you receive phone, you will check it for any signs of damage or wear and imei match. Only after you satisfied with item returned in same condition when sold, you are happy to issue refund.
Also I would demand to see copy of full documentation to back up what she has said and they also stated that there is no way this phone can be advertised as a NEW phone re your advert.