Can a property be sold after rectification by some other party?

"Person A & His wife " jointly had purchased a Flat no.705 In Mumbai with full payment having carpet area approx 595 sq.ft and also they had done a stamp duty & registration for the Flat. Now refugee area(Fire space) is also allotted on 7th floor due to that some carpet area of 705 is allotted for refugee area (fire space) and due to that carpet area of flat no. 705 is reduced from 595 sq ft to 450 sq ft. Since carpet area of flat no. 705 is reduced due to refugee area (fire space), builder is doing rectification of registration done for flat no. 705 and will assign some other flat to "Person A & His wife” behalf of flat no. 705 having same carpet area i.e 595 Sq ft. After doing a rectification a builder is going to sell the flat no . 705 to me. Is the process of selling a property to me after doing a rectification with Party A and shifting the PArty A to some other Flat is correct What are the correct process and document required in this regard so that i am safe in future