Builder not giving me refund against cancellation

Hello All, I had booked a 3BHK flat in Bangalore. The builder is a reputed Ltd company. due to unforeseen circumstances I had to cancel the booking. The builder told me that I need to wait for my refund until he finds another prospective buyer. though the agreement does not specifically mention this. I orally agreed to this and immediately moved out of the country and its been almost 1.5 years since my cancellation. the builder came back only recently and told me that he has a buyer and will refund me as and when the buyer pays them. The builder has refunded only 50% of the amount as of now and is making me to run from pillar to post for the remaining. my old aged dad is pursuing them on my behalf. The builder has started giving all sorts of reasons for non payment and also has told us that if we put too much pressure the new buyer might back out and we may have to wait for another buyer. In the meanwhile our sale agreement date also has passed. We donot know what to do next. Can somebody please advice us? Thanks in advance. Vijaykumar