Tax for land sale after price rise

The taxable income for the seller will be considered as the value on which the Property is finally registered irrespective of at what amount you got in the agreement of sale. 

So, if you have registered the land purchase at higher (new) rates, definitely the seller will have to pay at the new rates. 

As a buyer you need not worry about the vendor's tax liability.  Whatever the vendor has to pay as a tax will depend on the sale consideration price which is mentioned in the registered sale deed document.  Your liability is extinguished there when you have settled the sale consideration amount in full, the vendor has no rights to ask you to pay him the super tax or meet his house hold expenses.  Ignore it, if he is insisting on it, ask him to approach the court for recovery

thanks very much for valuable comments. Keen to end the transcation of sale with the seller in a happier way,  we're  trying to do our best from our side.