Normal procedure
Get earnest deposit (range from 1~3%), S&P signed take 10%, house delivered 100% collected.

At least get some earnest deposit before sign anything to show the sincerity of the deal, and draft the S&P clearly stated earnest deposit will be forfeited and penalty of 10% is imposed if money cannot be paid within x weeks/months time and S&P is deemed null and void after the time period.

But if buyer indeed is not able to fork out the deposit (even the earnest deposit) to buy, highly loan may not be approved as well...