How to protect basic pay in ONGC when joined from lower to higher post

Sir, I joined from one CPSE(NTPC) to Other CPSE(ONGC) through proper Channel. I was in the same scale & post during my interview at ONGC as that of NTPC. I submitted the resignation letter after receipt of appointment letter. However, during my resignation notice period of three months, I was promoted to higher scale/post. My service certificate mentions my higher post & scale. Now, ONGC is telling that since you joined from Lower post to Higher post , your basic pay won't be protected. Please provide some guidance in this case.