Seller not providing original sale documents what action can I take

Dear Sir, I am purchasing a newly constructed house, constructed an year ago. Total area is 1040 Sqft and Vendor is selling half of the house to me 520 sqaure feet. This is southern portion of the house and this has two floors Ground Floor 1BHK and First Floor 2BHK. I have already initiated the sale agreement & paid the downpayment. Next week we are going through the registration of this property. Now the vendor is saying that they will not handover the original Mother Deed & subsequent prior deeds to me,since they are retaining half of the house. They can only photo copies, please help me