If the sale agreement had a stipulated period for completion of the transaction in your case 6 months, then the buyer should have strictly adhered to the terms, if he has failed to come forth and register the property into his name by paying the balance even after the lapse of 6 months, he is barred from doing so thereafter.
He cannot dictate terms to you by stating that you should collect the remaining payments from individual buying each individual site or plot from him.
You can cancel the sale agreement with the buyer and forfeit a part of the advance amount paid to you and return the balance to him as per agreement of sale. Once the Sale Agreement is cancelled, you are free to deal with the property as you like.
Regarding your very important question, can the buyer ask or insist upon you to register half of the property as individual sites that he has sold or would sell in the future and collect payments from each individual site purchaser, this is possible or permitted if you think that your money will be recovered at least in this manner, but you must take it in writing from the builder/developer of these terms by clearly emphasizing that each site or plot can or should be registered only with you signing the absolute sale deed as a Consenting or Confirming Party and further emphasizing that any payment to be made at the time of registration your share must be paid separately by way of a Demand Draft.