Agreement for sale of land

Two agreements made on same day i.e 31.7.15. one for guideline value for rs 25 lakhs , which is lower than the the 2nd agreement which is at higher market value for rs 90 lakhs. property in tamilnadu. time to be registered by the buyer is 6 months from date of agreement. received 40 lakhs as advance. but buyer failed to pay balance amount and register within the completion of 6 months i.e 31.1.2016. the guidline value agreement for rs 25 lakhs was registered whereas the other agreement for 75 lakhs not registered. the terms of agreement in both are the same except for difference in sale consideration. one of the terms is if the buyer fails to register before the stipulated time of 6 months then he will loose the advance paid. if the seller received the amount or does not agree to register within the time of 6 months when the seller offers , the seller can deposit the money in court and claim for registration, now the seller has not paid the full money and time of 6 months lapsed. buyer is given notice saying that he approached with money but the seller are evading from registration. buyer asking to receive the balance amt i.e lower value and register the land. otherwise will file suit for specific performance. question ; which agreement for sale will hold good in court. the unregistered agreement for sale for 75 lakhs can it be stated in our reply to their notice. he has paid 40 lakhs but not given in cash in different instalments but seller not given any receipt for the same. kindly claify and guide us how to proceed further. what is our recourse so that we dont loose the property for a meagre value which is statedd in the registered agreement for sale. sundar