Delay In Delivery Of Possession Of Flat

My father booked a flat in May 2017 and the builder had given verbal commitment of possession by December, 2017. During this time, the builder claimed that 80% of the work is done. The builder pressured my father to pay the amount decided and to do so, my father sold his other flat, a farm plot and agricultural land all for a value much lesser than deserved, and paid 85% of the total payment by January 2018. The building did not make any progress at this time. The assets that he sold approx. 2 years ago, would have fetched him almost 30-35% more money if sold today. Had he invested in another property in the same area, the property rates would have benefited him more as they have shot up with an 18% rise in just 6 months. The possession does not look to be possible even by September 2019. We have to go for a rental accommodation due to this situation which will have a further cost impact. The loss of time, investment, valuable property and mental stress caused is tremendous. Please advice as to what compensation can be demanded in this situation and what kind court proceedings can be initiated if the case becomes any more complicated.