Gst bill from builder

Hi, Have recently purchased flat in Bangalore and need advise as mentioned. • When was purchasing flat, that time the project was about to finish, and builder promised in two months it will be completed? • Basis on builder’s confirmation booked and purchased the flat. • Have paid 80% of the value as per sale agreement through HDFC bank loan. • Builder said to pay balance 20% at the time of registration. • For six months builder used to keep saying by next month hand over the position. • Finally, builder asked me to complete the registration and I can start move in, as other people are starting moving in. • The Project was still not been complete, and it was going on… NO OC obtained. • Have did the registration and paid balance 20% as per sale agreement amount and paid 5% GST as well, this was also as per sale agreement. • However, after I paid builder gave me handwritten receipt for the paid amount, since have paid GST, I was in demand to get my original tax invoice with mentioning the GST amount. • Initially builder said he will provide original tax bill in a weeks’ time and later on he started ignoring this. • Have moved in without getting OC and yet to get original bill. • Need an advise will I get original bill for the paid amount to builder? • Is it illegal that have moved in without OC? • Now how to get rid of this and how to catch hold the builder? Please advise.