How to check validity of agreement of sale

Hi Sir, We have had an agreement of sale with landlord in Mar-2014 for 4.5 acres of land and paid 25% of total agreed amount to the landlord. As per the Agreement either we have to sell the land to third party and give the rest of the amount to landlord if we fail to sale the land in stipulated time i.e. 6 months then landlord agreed to register the portion of land for the amount that we paid at the agreement time. We are not able to sell the land in stipulated time and we later found that the said land is an assigned Land. Now the landlord in denying to return our agreement amount or doing registration of portion of land (as it is an assigned land to the owner by government)as agreed on our name. Please advise how we can proceed with it to get our money back. Can we approach legally or what are options to get our money back from the landlord. Is their any time limit that agreement will expire as we did not register the agreement document.