property issue

Hello sir...
In 2016 we did a agreement with some one for land purchase in 100rs stamp(hand written) & paid half amount(9Lac) in presence of 3 witness & they did sign in that paper, now seller doesn't want to sell the property so what is the legal action we can take? is that stamp duty fine is applicable or what? & is stamp paper is valid or not because it's in 2016?
please tell me the pro's & con's.....