Aster Hospitals Jp Nagar Bangalore - Excessive charges for treatment

This is in regard to my ** year old Dad, Mr Jagannatha Sharma (UHID: *********) recent admission and treatment at Aster RV hospital in July.
My dad suffers from Chronic Liver disease (NASH) and had undergone TIPSS procedure in June at HCG hospital whilst being under the care of Dr. Veerendra Sundur (Medial Gastroenterologist) at his private practice. On July *th my dad developed hepatic encephalopathy and was rushed to your hospital emergency department after which he was
admitted into the ICU to manage the condition (high ammonia). My dad was accompanied by my mother who was not asked about the place of work (KPCL) during his admission.
While he was admitted and was taken care of by RV team of healthcare professionals, I was still in
Australia, unable to travel and was in touch with their billing department via phone and Whatapp. I transferred funds to the account number that was given to me via NEFT regularly.
I want to bring to your attention the fact that my dad is a retired employee of Karnataka Power Corporation Limited (KPCL) and was/is eligible for CGHS cash rates, as confirmed by multiple employees within Aster RV.
However, my dad was charged under the ‘cash rates’ for the treatment he completed in two admissions. I requested the billing assistant Manager to re-work the bills to match the category and tariff that my dad should have been billed.
While I was assured that this will be discussed with the finance team, I was told that there is no way that a ‘closed bill’ would be re-opened/re-worked and it was the patient/carer’s fault that it was not declared at the admission time. I
wish to clarify that my mom was not asked about my dad’s previous employer which woud have confirmed that he is eligible for CGHS cash rates. My concern is that this was never asked, and I believe if was asked all the bills would have been at CGHS rates.
I request that the Aster management be asked to apply the CGHS rates for my Dad’s two admissions and process this by issuing a refund. I believe it is something that will be possible in good faith.