Nirmala Hospital Pharmacy — Selling medicine without Tax Invoice and not taking return of sold medicine

Today 9/10/2007 Time 3.32

Last night I had been to Nirmala Maternaty & Orthopadic Hospital situated at Vijaya Nagar Colony Opp: Vijaya Nagar Post Office and Consulted to Dr.Ramesh (Orp Consultant) after consulting with Dr wrote some medicine to purhcase then with in the same hospital builing the Pharmacy is situated. There i purhcase two kinds of Tablets and One Gel Tube for external use.
the shop keeper didn't give me the tax invoice and he collect from me 174.00 . Once I reach home i found same combination gel tube and send my brother to return the Gel tube which I purchased with pharmacy. He shopkeeper is not willing to take his goods back and he is telling me to exchange the medicine with one other medicine and the behviour is very bad of this shop keeper.
My Question : This is valid reason which shope keeper is telling me without the valid tax invoice, He cann't take the stuff back?

With Regards

Mohammed Asad