Export bags - Cash not refunded

On friday **-*-** I bought a cash bag for Rs. *** from E. K. Bags, SM street, Moideenpalli road, kozhikode.They did not give any bill to me as well as to any other customers at that time. Due to some inconvienience in using that bag I returned it today (**-*-**) There was  no other bag to my need there.  still I took one bag of Rs. ***. Now they are not willing to give me the balance. They says I have to purchase any other thing for the balance amount. There was no other product to my need and I requested  for the balance amount which they resisted. I have to leave just by a quotation /estimate slip written -to give me any product worth Rs. ***/-. If there is no product for my need how will I get my cash back. So kindly help me to deal this.