LG Electronics - New Washing Machine Buyback not received

Currently I am Staying at Guwahati.
On **th June, ****, I bought one LG washing Machine under exchange offer from Great Eastern Shop (dealer/supplier of LG), Rukminigaon, Guwahati.
Once I visited the shop, I was informed about one exchange offer LG, wherein in exchange of old washing machine a buyback of Rs. ****/- be credited to PayTM account. I liked the offer and on further inquiry, I was told to pay for the new washing machine and was assured to get the old washing machine collected by the shop/LG, post which the buyback amount would be credited to my PayTM account.
Accordingly I paid of Rs. **,***/- for the new washing machine, which got delivered and installed during next one week.
But ironically, nobody from great eastern/LG came to collect the old washing machine. After lots of follow up with both the team, finally, after around month the old washing machine got collected by LG team.
However, the buyback was not credited yet. So again after series of follow up with LG/ PayTM team, after one more month I got one PayTM voucher in my PayTM account. But when I tried to redeem the voucher, the PayTM informs that this voucher need to be shown to the cashier at the shop and accordingly the amount can be adjusted against the new washing machine payment.
As I have already paid the full mount earlier, I contacted the Great Eastern Team, but Branch Manager (Mr. Saurav) had been vary abusive and categorically denied to take any responsibility and disagreed to extend any co-operation.
Thus I was harassed by the Shop/ LG / PayTm and faced the hardship narrated above.