Can the seller increase the rate of the plot against the one decided ?

The purchase of residential plot is registered in the registrar office by paying the proper stamp duty. I have submitted the index copy to the Talathi office for completing the process and to obtain a 7/12 on my name. Talathi issued a notice to verify the transaction and to certify the mutation entry. However, the seller is not signing the notice stating the reason as a dispute with me (buyer) about the money. Seller is asking more money than what was mutually agreed before the registration process. What should I do?