What is wrong with this as the control source for a field on a form?
=[txtVolume]*DLookUp("[BuyerSideRatePerUnit]","tblCommissionDefaults","[ProductID] = " & [Forms]![frmDealInput]![cboProductID].[Column(0)] And "[CompanyID] = " & [Forms]![frmDealInput]![cboBuyerCompanyID].[Column(0)])