Payment for property

I have paid Rs 1 lac booking amount to builder for a new ready to move flat with Occupancy certificate near Mumbai. The total cost is Rs 25Lacs. The builder wants me to pay the full amount to him before registration/stamp duty. He says I have to pay the full amount before registration as I am self funded and not taking a loan. Is this ok and should I take any precautions or insist on registration after part payment? The property documents are clear and he is giving receipts for my payments.