Buying Flat from owner directly

I am planning to buy a flat from the owner, 1) Builder (75%)-Owner (25%)sharing as per their sharing agreement. 2) Currently flat is around 75-80% completed, And OC/CC will be getting by jan-feb 2021. 3) Owner is selling the property now directly. and telling that he is do the registered agreement and take the 10% amount and then going for bank loan for 70% and then during the final registration rest 20% amount Question:- 1) Is it legal to do the registered agreement before getting the OC/CC certificate from the concerned authority. 2) SBI bank is telling that they can release the amount during registration only, So owner is telling that he can register now only that we can get the bank release amount. 3) Owner is telling that he can do the registration since he is the owner of the land and flat as per sharing agreement. is it legal to get the flat registered before getting the OC/CC or possession certificate. 4) what's the general process for buying owner share flat/Proprties