Missing: sale agreement

Hi Experts,

I am buying a flat. The seller has all the original documents which were requested by my bank before disbursing the home loan but the original Agreement between builder and seller is missing.

Seller has followings in original:

1.      1. Sell Deed

2.       2. Tax Paid Receipts

3.       3. Khata Extracts

4.       4.  Encumbrance Certificate

5.       5. Possession Certificate from builder

MISSING: Sale Agreement between Builder and Seller (1^st Owner).

How should I proceed with this deal now?

Thank you.