Tenant Breaking a lease

[Shitij Agrawal]

I am new to investing and became a investor by renting my own condo in Boston. I have a property in Boston and My tenant had renewed his lease and then broke within a month of renewal. He had rented the Condo on June 14 2018 for ending on June 30 2019. He renewed the lease in April 2019 ( for period of 1 year from Jul1 2019 to Jun 30 2020).  He sent us the letter for breaking the lease on July 13 2020 and According to my property manager that he understands his responsibility for 1 more year and we communicated that we will be holding him responsible only till we find another tenant. He paid us till he was in the property till Aug 2019. 4. He had last month rent and security deposit with us. It is still in escrow account. 5. We had communicated that he is liable for rest of the year. We have sent reminder for rent in sept but instead He sent a letter demanding security deposit .
Questions and what I am hoping for?1. How can I ask him to pay for the rent for remainder of the term? what are the odds with this? 2. Do I have to return the security deposit? 3. What are my role if I decide to sell the property instead of renting another term? Thank you everyone for guidance