Builder is asking for payment before registration

We bought a builder floor from one of the individual builder in June 2018. It was ready to move in property with some work that was required to be done, followed by registration in Aug 2018. We made a legal agreement between the buyer and the seller to pay 20% of the amount (15 lakhs) in June and rest of the payment to be done at the time of registration. We paid 20% of the total amount via cheque. Now the builder is asking for more payment in parts before registration. He is insisting on paying before registration because of his personal dispuit and if we don't pay in parts, he's ready to break the deal. In agreement, it's mentioned if the buyer breaks the deal, our 20% stands with him and if he does that, he will need to pay the double amount. I have 2 questions: 1) How safe is it to pay him in parts before registration? We will need to pay around 20 lakhs more before registration via cheque as rest of the payment will be coming via loan. 2) If he breaks the deal, what are our legal options?