Sale of Flat during Redevelopment in Mumbai; after DA but before PAAA.

The cooperative housing society in Mumbai in which my friend stays is going in for Redevelopment. The Development Agreement (DA) with a Reputed Builder has been signed and registered, with the Builder, Society and all the Members of the Society as parties to the DA. As part of the DA, some Corpus (Hardship) amount has also been received by my friend. The PAAA (Personal Alternative Accommodation Agreement) is not signed yet but will be done in a few weeks. After PAAA, the new building will be constructed and my friend will get a new Flat with larger area in return for his old Flat. He will also get a Corpus Amount, Rent etc for the period of construction, till the new Flat is handed over by the Builder. At this stage, my friend has received an offer from a close trusted relative to buy his old Flat. My friend would like to sell only if it is strictly legal to do so and only if there will be no income tax related issues. My friend absolutely doesn't want ANY Legal or Income Tax issues either for himself or for his relative, whom he considers as his own brother. Some friends advice that because DA is signed, my friend is no more the owner of the Flat, so he can't sell the Flat. According to them he has already sold his Flat to the Builder by signing the DA. He can possibly sell his right to the New Flat; but the builder will have to be a party to the Agreement. It is also not clear how the Capital Gains Tax will be calculated. Is it Long Term CG Tax on date of DA? Then what about the Sale of Rights by my friend? Is it legal? Will it attract what CG Tax? So kindly advice please. Is it possible to go through this Sale Process, without losing one's peace of mind over Legal & Tax problems? Are there precedents for such Sales or IT Department can raise a lot of legal & tax queries? Kindly advice. Would you recommend FOR or AGAINST such a deal, if strictly No Legal or Tax Issues IS A PRE-CONDITION? Thanks and best regards.