City Union Bank — Mortgage letter not released after paid the loan amount

Address:

Nagercoil , Kanyakumari District , Tamil Nadu.

I have given my house to my father in law to take a loan at city union bank. Unfortunately he was passed away in an road accident, therefore i have received a bank notice from City Union Bank to pay the outstanding loan amount of what my father in law took from the bank.
He pledge 2 houses which is my house and his own house, I have approach the bank and asked to release my house only by paying the amount which took against my house. My house was kept for 3500000 RS however the bank asked me to pay 4500000 to release my house.
I have requested several time to reduce the amount as the current value of the house is not worth for 4500000 RS however the bank didn't accept. Then I have paid the said amount of 4500000 RS on 16/04/2020 during this corona pandemic asked by the Nagercoil branch Manager He promised that he will give back my land documents and do the release letter (Mortgage letter) from the Register office once I pay the money.
After I have paid the full amount he has return my house documents and he said he will release the mortgage letter from the register office within a week. From that day I am asking him to release this mortgage letter which he keeps postponing till date .
As i paid all the agreed amount I have to get my all documents however till date I didn't get the mortgage release from the register office.