Rent related

The owner did an agreement with a tenant through an agent to give his flat on rent. The copy of agreement is attached. The rent was starting from 15th March 2020.The owner was living abroad so through that agent he advised the tenant to collect the key and take possession. The tenant requested that he was unable to shift due to Corona curfew. He will shift in the flat as soon as he gets a chance after removal of the local curfew. He also requested the owner to start the rent when the curfew would be removed. The owner agreed. The three months passed. The owner checked with his local friends and found that the curfew was already removed so he called on phone to the tenant to shift in the flat as life was normal in the city. The tenant replied that he had already moved from Mumbai to Calcutta few days before. The owner was not intimated when he moved and only disclosed when owner asked him to shift in the flat. Why did he not shift into the flat once he was able to travel by train from Mumbai to Calcutta? Then owner requested to catch the train to return to Mumbai and shift because of normal life and rent would be started since this date of intimation. He refused to shift and said that the owner could look for another tenant. The owner asked him to send written letter for the cancellation/termination of agreement. But he did not send. Then the owner again reminded that the flat could not be rented to another person until he cancels the agreement in written (Para 9 of agreement). Still, he did not send signed agreement cancellation letter to the owner. Then the owner drafted an agreement cancellation/termination letter and sent him by email on 22 June 2020. And the owner requested to sign and send the scan copy so that after this he would look for another tenant. Also informed that the rent will be deducted for those days which will be delayed by the tenant to sign the agreement cancellation letter. The tenant did not reply even several reminders on phone. Then the owner waited until 12 September 2020 and refund his deposit after the deduction of half month rent. The owner exempted three months rent on humanity basis from 15 March to 22 June 2020. Then from 22 June to 12 September (82 days) was rented time as explained before. But the owner again exempted 67 days rent and charged only for 15 days. Now the tenant is asking to refund that 15 days rent and keep on calling and harassing and threatening to complain. What the owner should do? To refund that 15 days rent or not? Please advise based on attached agreement paper and the situation explained. Thank you,