Two agreements to sell with different values

I signed 2 agreements to sell. One for FULL value and other with 30% less value - as seller wanted to save capital gains. The sale deed got executed 2 months ago acknowledging full & final payment. After 1 months the seller has come up with the 2nd agreement and asking for MORE MONEY based on 2nd AGREEMENT saying it was executed on our request to save on STAMP duty. Since i actually singed both the agreements on the same day - should i worry or should i deny one of the agreements saying it was re negotiated for some reasons. As the 2nd DOCUMENT of higher value is an illegal document - can the seller take any legal re course on the basis of illegal document - 2 months after execution of the sale deed. If i say that 2nd document was re negotiated - then can he use the whats app chat which has extensive discussions and can prove that the 2nd document actually exists. 1. Can a case be admitted on the bases of illegal document after sale deed 2. Can whats app chat decide the case