Sale deed

I'm planning to purchase a empty land residential property. The overall square feet of the property is 1953, from that 1050 sq. ft was sold already in last year. I'm planning to buy the remaining vacant land. For that the seller is saying he won't give the parent document to me. He is planning to give the patta copy to me and the parent document to the 1050 sq. ft buyer.. Is it good to buy or parent document is mandatory. Kindly share your valuable suggestion.