Field Revie Rebuttle

Greetings fellow investors... I have a situation where one of my investors are purchasing a property where as in order for the buyer and seller to close the deal than the buyer would have to purchase the property at the Market Value. An appraisor appraised the property @ 150K. The buyer and seller are content with this and there ready to close. But the Field Review came back with a Market Value of only 130K. I was informed that the Field Review can be Rebuttled or we can either order another Field Review from someone else. Please add on or just give me some advise in this matter. Any knowledge is appreciated. ;)