Apollo Pharmacy — Denied to return product.

I being a regular customer of apollo pharmacy-anna nagar. I have purchased a similac stage -2 baby food for my 1 year old boy around mid or last of july-2020 (Exact date no idea as i don't have the bill). But due to some series of problems faced by me i was not able to return the same to the shop. Then as on 11-sep-2020, i went to return the same, but they denied to return it and the packet was sealed and was not expired as per the date mentioned on it. I asked them to exchange but without giving proper reason they denied sometimes they say i don't have bill or they say its being 3 months etc. I asked them to give the proper reason for non return in writing. But they clearly denied. Kindly help.