Mansuri Communication - Didn t gave the bill and now not accepting the flaws in the product

On **th April, I went to Mansuri Communication at Arjun Park, South West Delhi to buy an Aux. He gave me the aux for **rs and DIDN'T GAVE ME A BILL. When I came back home I checked the aux but it was not working with my phone though the other oenes were working. I didn't went to his shop at that time because it was very far from my house and also at **PM night curfew was announced by the govt. So on **th April I went to his shop again and told him that the aux wasn't working but he refused to give me the money and another aux. He also checked that with his own phone but still it wasn't working. His shop is in Arjun Park, West South Delhi. I hope you'll solve my problem ASAP.