Can seller request buyer who is financing with a VA loan to sign a apprisal waiver?

The Appraisal Waiver is not to be used for VA or FHA financing. Look under the heading of the form. The 3rd Party Financing for VA addresses this issue and says Buyer can proceed if appraisal is low but has to bring non borrowed cash to cover difference. I would refrain from verbal agreements but you could let buyer know that Seller will not reduce price if appraisal is low.