Regarding ownership of Flat build on Refugee Relief /Colony Land

Dear Sir, Warm Greetings of the Day, I am intended to purchase one 2 BHK Flat at South Kolkata Baghajatin Location. Sir, said flat has been developed by the builder on a land called as Refugee Relief Land / Colony Land (i.e. these lands are gifted by WB Govt. to migrant from Bangladesh after 1947 partition under condition not to sell ). Sir, said property builder has got power of Attorney from land owner for building and promoting the flats but Plans for those flats are not sanctioned by KMC due to nature of land/property. Sir, builder said that the property has been sanctioned by local councillor & can be sold or purchased without any legal hindrances . Builder is also assuring that the proper mutation will be done against the name of owner of any such flats & individual property Tax will be charged by KMC (Kolkata Municipal Corporation) against the ownership of each property. Builder can provide me with maximum numbers of documents related to say property: 1. Master Deed (Dalil / Daan Patro) 2. Promoter’s agreement with Land owner / Power of attorney 3. Land owner’s Mutaion Certificate / Tax payment bills 4. Assesment Role Copy Sir kindly guide me how safe is this for me to purchase such property said as above considering future legal complexity regarding complete ownership related authority/permissions. I will remain great full to you forever for you kind guidance & suggestion.