Employer is not relieving after served the notice period & paid retention amount

5 years ago

I'd joined the organization at 18 Oct 2016 with two years of service bond but because of so much work load, not personal satisfaction & got the better opportunity, I'd put the resignation on 19 Jul 2018 & served the three months of notice period. I'd lots of convinced to them for accept the resignation & relieving me via mail, call & face to face discussion & finally exited from the oragnization on 29 Oct 2018 after the not getting any positive sign from them. I've also completed my two years of service with organization.
They denied to relieve me because of put down resignation before completion of two years of service bond but as per clause mentioned in appointment letter, employee need to pay retention amount if leave/ resign from organization before completion two years & I've already paid retention amount to them but still manager regret to give relieving letter, retention amount & also not give last month salary to me. They treat me as a absconding case.
I have the resignation discussion proof.
So, please help me. What can I do for in such case