Validity of Sale Agreement

I had entered into a sale agreement with a builder on June 2012 for purchase of an apartment . The agreement had the schedule of payments proportionate to stages of completion of the project. The completion date was mentioned as 1 year from the date of agreement. However, the builder requested me to get the property registered during March 2013 and i agreed. Since the project was through only 50% during the time of registration, i informed the builder that 12.5% (Rs. 4 Lakhs) of the agreed cost will be paid after completing the project. it was agreed and a post dated cheque was also given. The property got registered. As there was no progress and commitment from the builder in completing the project, i did not honor the cheque. Finally, the builder completed the project on March 2015. He is following for the balance and i am in the process of arranging it. Meanwhile, he has sent me a letter to pay interest of Rs. 3,73,000 for delayed payment. I request you to kindly advise me on how to deal this. Also, let me know whether the sale agreement is valid after the sale deed is made?