Capital gain tax benefit without registry and only transfer

Dear Team, My father has sold a plot in Haryana (after 8 years of possession) and now to save capital gain tax, he is planning to invest the gain amount in a flat in Noida with me and my wife as partners. There is a property where the builder has dues towards Noida authority and thus the registry is not being done and the owners have just the possession letter with them. As there is no registry done, thus no bank is offering loan on this property. We are trying to somehow manage to buy as the rates are less due to all these concerns but we have a query that when there is no registry done and this will only be a transfer in records of Noida Authority, will my father be able to get capital gain tax benefit or that registry is mandatory for this. There is a case filed by residents in the court for registry as they have been residing here for last 5 years and there is no definite time period by when the registry will be done. Shall we buy this property and will we be able to get the capital gain tax benefit?