Banakhat vs. dastavej

Hi All,

Please guide me for following query:

Recently I had a  deal with flat to purchase in resale. I have given token amount and rest will be given by loan and other way

Currently the building is under builder's head and not transferred to the society.

Builder suggested me to do a banakhat and give other amount (except for loan amout) to the seller and seller should give possession at that moment.

Then give loan amount at the time of Dastavej.

Following are my queries:

1) Is Banakhat mandatory for resale flat? 

2) Is Banakhat valid to give other amount and take possession? What if tomorrow seller will claim that I have not sold my flat.

3) What time duration should be keep between Banakhat to Dastavej?

Thanks