Reliance Mart — Forcing to buy products at security

We were buying an 1+1 offer product, after reaching the exit the security officer told that there was only one product in the list and it was of half price, i told them that's okay, but they forced me buy the other one and kept the bill of that saying they need to keep it for the records . Was it okay to force someone to buy?
They need to be customer friendly and make us understand the issue .