Exemption from long term capital gains tax

Anand (Avp) (67 Points)

02 September 2020  
My mother sold a flat in her name on 30th Aug 2028...In the process, there was capital gain.

Understand, this year, deadline for claiming capital gains has been extended upto 30th September 2020 as against 30th Aug 2020 under normal conditions.

Now, we are purchasing a ready possession (OC received) flat against the capital gains in my my mother's name. Agreement registration + final payment shall be completed before 30th September 2020.

Question is
,Whether we need to get builder's possession letter before 30th September 2020 or only registered agreement with full consideration paid is sufficient to claim LTCG exemption?