In the first instance, I'd transfer the balance (after deducting fees) into a separate account......I would say one offering high rates of interest but there don't seem to be many of those around.....and just let it sit.
Your potential purchaser may pop up at some point in the future and want his money back and you'll be able to do this without too much inconvenience if you have kept it separate from your own monies.
However, if you feel that you have made sufficient attempts to contact him, re-list your land for sale.