what was the term in the agreement? Was it mentioned that he should pay with any specific date the full amount and get the sale confirmed or that the sale agreement shall not be enforable after any date? Was it mentioned that the advance shall forfiet upon the expiriation of this agreement ?
Can you give the details ?
However you just don't pay attention and tell him that the agreemnt is expired and not enforceble,if he want to purchase the land he should give presant market value.
The worst to worst he will go to court ,you should get a good lawyer and contest the case,in court demand the presant market value.