1) once you have entered into agreement for sale you are bound by terms of contract
2) as per said agreement you were required to pay Rs 11.66 lakhs for purchase of land in 6 months . admittedly you have paid him Rs 4 lakhs and had offered to pay balance amount within 6 months period
3) if seller has refused to honour the agreement send him legal notice to execute regsitered sale deed in your favour and offer to pay balance amount
4) if seller refuses to accept the balance amount and execute registered sale deed in your favour file suit for specific performance of the contract . obtain injunction restraining seller from creating third party rights pending hearing and final disposal of suit
5) seller cannot arbitrarily increase the price .
6) if however you have failed to pay full amount within stipulated period of 6 months it is better to amicably settle the dispute with seller