Fortis Hospital - Hospital overcharged for planned angioplasty without proper invoice

My father was admitted to Fortis Hospitals Bannerghatta on *.*.** for an angiogram after which a couple of blocks were detected in his heart. The doctor had suggested for an angioplasty and he was admitted for the same on the same day. The estimate given by them was Rs. ****** (for private bed) + Rs. ***** (for * stents) + Rs. ***** (for * balloons) = Rs. ******. Everything went well and he was due for discharge on *.*.**. He was insured and the insurance approved an amount of Rs. ******. The shock came when the billing dept. informed me that I had to pay an additional Rs. ****** to clear all my dues whereas the billed invoice was only of Rs. ******.**. Ideally I would have required to pay Rs. ***** (amount deducted by insurance). This extra amount is not present anywhere in the invoice and they kept on telling me it's because I had opted for a private bed whereas my insurance had approved for a gen ward. In reality the difference in estimation for angioplasty between gen ward and private bed is Rs. *****. I had no option but to pay the amount as it was quite late in the evening and they wouldn't have discharged my father otherwise. This is how Fortis looted me. Despite multiple demand for explanation I wasn't provided any from the billing or insurance desk. I want Fortis to refund my entire amount and also compensate me for the harassment.