Vat in case of builders

Dear Professional Colleages,

In case of Builders taken permission under Lump sum payment of VAT (in the state of Gujarat), when to pay VAT? at the time of receiving member bookings/advance amount or at the time of final sale agreement after completion of Scheme/project. Builder is issuing only receipts to its members for the payment received. No Agreement to sale/Banakhat is being executed in favour of member.

Your valuable reply will help me in serving my clients better.

thanks