Refund of booking amount -Shantee Builders

Respected Sir, On 12th Dec'2015, I booked 1 BHK flat on 6th floor (underconstruction) and paid 10% booking amount which Rs. 1,94,250/-. I was told that the registration would happen after a month since the partner was out of town. In the mean time, my band asked me to provide supporting documents in order to take loan hence I provided the same. Later, I came to know from my bank the builder does not have CC and approved plan for 6th and 7 th floor. I contacted the builder over the phone and they kept saying it would take a month. I waited till first week if April and asked for a refund since the sanction letter was about to expire. The builder called me to his office and asked me to give in writing that I want to cancel. Hence, I wrote the same and handed over the letter. I also took their acknowledgement on the copy of the letter. Next day they called me in their office, just showed me the cheque and asked me to sign on NOC and receipt of cheque. I did as asked. The name and amount on the cheque was correct. I happily collected the cheque and deposited to my bank. After few days, I came to know that I was issued a post dated cheque dated 10th June 2016. I realized that I didnt cheque the date while collecting it from the builder. The builder is not entertaining me now. Please help what should I do since it is my lifetime savings.