sale agreement

A enters into a sale agreement with B for sale of a plot of land to B against payment of sale price of Rs.320,000/- to be paid in Rs.20,000/- as down payment and remaining Rs.300,000/- in 30 monthly installments of Rs.10,000/-each by B to A. On full payment of sale price of Rs.320, 000/- in agreed period of 2-1/2years by B to A, A will execute a sale deed in favour of B. A gave possession of said plot of land to B on down payment of Rs.20,000/- by B in good faith on firm assurance by B to pay monthly installments regularly to A. B constructed a house on the said plot later on.
B defaulted to pay monthly installments regularly and despite reminders by A, B paid only Rs.100, 000/-in 30 months to A. After 30 months A cancelled the sale agreement and demanded possession of the plot from B. My queries are as under:
1- Is A right in his action of canceling the sale agreement? Is giving possession to B a bar in cancellation?
2- If A allows B to pay the remaining sale price in further 12 months but with a penalty of Rs.30000/-. Will A be right in his action if B agrees to the offer?
3- If B defaults again in payment of his dues, will A be right to cancel the sale agreement again and demand vacant possession of his plot from B.
4- If B files a law suit against A for specific performance of the agreement on grounds that: a)-He has possession of the plot and constructed a house thereon by spending a lot of money, and b)-ready now to pay the remaining amount of agreed sale price. Will B be entitled for degree against A.?
Please provide section No. with relevant LAW or rulings by High/Supreme courts. THANKS.
From: M Murtaza Email: murtaza1936@yahoo.com