Seller refusing to do sale deed after agreement to sale

We have purchased independent ground floor in a triple story building we did agreement in June and paid more than 10 percent amount at time of agreement and rest to be paid at time of sale deed and last date on agreement was 30 September 17. As property was not complete he started asking for money to renovate and complete left work. Then he started pressurising us for more money and then we told we will not pay until you complete work and will pay only at time of sale deed. Now 15 days to go he says give me 2 lacs more than agreement otherwise I will not do sale deed. Please advise what legal action and preparation we should do before hand only as last date we will appear in registrar's office on 29 September but still want to know what else we can do before hand to have strong case against him and his harassment.