If your most qualified prospect submits a Letter Of Intent that includes a lower price and more generous terms than you had wanted or a less qualified buyer has submitted a more attractive LOI, then now is the time to negotiate with the stronger prospect. It's almost impossible to improve the deal once you have accepted (by signing) the Letter Of Intent. However, the buyer may find reasons to re_negotiate a lower price during the due diligence phase.
Of course you may add or subtract items as needed or change the time limits for your particular transaction. This little contract clause may save you from making a huge mistake down the road, so be sure to save this one for future reference!