Expert opinion on resale of under construction property by builder

I have a 1 BHK apartment that I booked in 2013 with Trivandrum builders in Thiruvananthapuram. The value quoted by builder in the apartment allotment letter was 19.75L with estimated completion in 1.5 yrs. Builder has not executed any stamped agreement with me and has not given me any construction status linked payment schedule; however he has delayed the construction as on date. Builder entered into an agreement with landowner and has paid only 3L out of total amount of 2.25C, due to which landowner has registered a legal case against the builder. I have already paid about 85% of the total money for the apartment construction but the builder has not completed the work inline to the amount paid. Now he is pressuring me to release another 10% of the remaining amount, without which he informed that he will sell the under construction agreement apartment to someone else quoting me as the payment defaulter and invoking appropriate clauses in agreement. I am seeking legal opinion to understand how to proceed on this issue - I have 2 solutions in mind, one is to ask the builder to complete the apartment and get the pending amount on apartment registration and second is to ask the builder to settle the amount invested by me in the apartment along with interest for the duration. How should I proceed, please suggest