This is not the way a property can be purchased.
Firstly have you seen the property physically and have verified the details and have satisfied yourself about the authenticity of the documents as well as the property itself. ?
There is no such system in India that the property papers shall be franked in the registrar office itself which can be considered as a registered document without you physically present before the registrar during registration.
For registration of sale deed or agreement you need to be physically present in the registrar office or else it can be considered as an act of impersonation and held invalid and illegal.
What is the necessary to knock the back door when the front door is kept open ajar.
There seems to some suspicions about this deal, you may lose your hard earned money if you are getting lured and hurrying up due to this , better be careful and handle patience to verify the veracity and confirm the genuineness before getting into such agreement.
If the builder says that there was no need of any ID proof, PAN card , current residence proof etc. for the sale agreement then ask him to do the registration without the payment, produce the registered document and collect the amount.