Amazon .in - REFUND NOT RECEIVED

I ordered A POCO F* by Xiaomi Graphite Black, (* GB RAM, *** GB) Order # ***-*******-*******through Amazon and received it's delivery on **.**.****.Due to some personal reason, I could not open the product on due date (i.e. **.**.** return or replacement allowed date). Actually I bought this mobile for a surprise gift to my wife on her birthday which was on **.**.**. But when I open the box, I got surprised that product is second
hand. I paid for new mobile and the seller supplied me the old mobile (old invoice dtd **.**.**** was there with the name of Ankit Patole). I informed to Amazon and with lot of efforts and multiple mails finally amazon has accepted the fault of seller and requested me to kindly get the seller’s address over written mail and then returned the product directly to seller. Amazon also instructed me to share the tracking no. and charges of return post so that they can track and refund the money including postal charges.
as per amazon instructions in written mail, I returned the product to seller on his address directly with ref no. EC*********IN. Item was dispatched on **.**.** and delivered on **.**.**** (As per the written confirmation over mail by Sr. Supdt. Of Post Offices, New Delhi Central Division, Meghdoot Bhawan, New Delhi ****** and Karol Bagh PO.) I shared all the proof to seller as wel as Amazon but still they are not refunding my money. Amazon is saying that until and unless they will get the written communication from the seller that he has received the product they will not issue refund I already requested to seller also over phone no. ********** to kindly revert via mail to Amazon but he is not doing so. In between these communicationss, * times (dated **.**.** & **.**.**) amazon has confirmed over mail that refund has been initiated but refund is not received till date. Despite of their tag line that Customer satisfaction is priority Amazon is misguiding me. Please help to get my money back.