Employer not returning original certificate

Hi, My wife was working as a Librarian for a CBSE school in Bangalore. Joining time she submitted all original educational certificates at the school. She resigned recently and had to join the next company within 20 days. Hence we paid 2 months salary to compensate the notice period. School management was not willing to give resignation acceptance & Relieving letter & Original certificates. They said the reason as missing books from the library. Since the library & school was opened after almost 2 years of COVID lockdown there was no stock verification done. No documentation regarding the library or missing books were handed over during the time of her joining. Stock verification has been done just 2 days before her resigning for namesake. For which we couldn't submit a report too. They immediately appointed a new Librarian without accepting our resignation. Now they are again demanding 40000 for the missing books, then only they will release the original certificates. We demanded for a re-stock verification since we are sure that the book missing happened before her joining or there is no real missing scenario as such. We would like to know the legality of employer holding back the original certificates. We don't need the experience letter/resignation acceptance letter which they can issue us once the re-stock verification is completed. Our only requirement is for the original certificates which I believe they dont have any authority/right to hold . Please suggest how can we proceed in this matter.