Whether unregistered sale agreement valid after 3 years

We have entered in to an unregistered agreement on nonjudicial stamp paper for consideration price of Rs 0 with a stamp duty paid for Rs1000/- on [deleted] to sell agriculture land of area 9A10G consisting of 7 sy nos and is not registered.The agreement amount of sale was Rs 18.75 lakhs per acre and the total sale consideration was Rs 173.4375 lakhs , out of which Rs 50 lakhs was recieved as advance and the balance 123.4375 lakhs to be paid after registration. The agreement period was 4 months. Out of theabove 7 sy nos , one sy no measuring 30 G was under litigation and another sy no measuring 2A was having part no and full no has to be allotted for this. There was a condition in the agreement that the buyer will get registered the clear 5 Synos within 4 months and the remaining 2 sy nos after the court case is cleared for one of the sy no , and whole sy no is allotted for the second sy no. But till now the buyer has not got any of the lands registered and has not paid money, even after regular oral persuations. He is giving one or the other reasons.In this situations i want clarifications as follows. 1. Can i go ahead and sell the land for other buyers? 2.Is the agreement still valid even after 3years and 8 months? 3.Is there any legal constrains to sell the land? 4.What legal course the buyer can adopt? 5.Do i have to refund the advance money, even though i have suffered huge losses by the action of the buyer by not honouring the agreement in time. Kindly someone guide me since i am a law abiding as well as a god fearing person.