Assessment is only for tax purposes, not involved in the sale. If he meant appraisals, the loan or appraisal should be a contingency of every purchase contract, but is an option that buyers sometimes elect to eliminate in sellers' markets -- a mistake in my opinion. On a low appraisal, buyers are expected to make up the difference in an increased downpayment if they do not have a loan/appraisal contingency to protect them.
Truth is, almost any purchase contract can be broken regardless of whether the loan/appraisal is a contingency if the buyers want out bad enough. If sellers won't let them out, the home can be tied up in an open escrow for a long time with litigation pending, and usually the most the sellers will get is the buyers' goodwill deposit. During this time sellers cannot accept other offers except as contingent upon the successful cancellation of the existing escrow (which could take months unless sellers agree to return the original buyers their goodwill deposit).
N2B