How to substract Balance from RequiredAmount in sql query

Question

Proplem

How to substract Balance from RequiredAmount in sql query .

when write balance on first select it not defined 

actually i need in first select Balance(alias)inside union - RequiredAmout in WahInvoice Table 

select BranchCode,UnitCode,RequiredAmount, Serial,Year,Month,CurrentReadingDate,CurrentMeterReading,LastMeterReading,CurrentConsumption,CurrentConsumptionValue ,VATValue,CleaningFees
from
(
select BranchCode,UnitCode,RequiredAmount, Serial,Year,Month,CurrentReadingDate,Convert(varchar,CurrentMeterReading) as CurrentMeterReading,Convert(varchar,LastMeterReading) as LastMeterReading
,Convert(varchar,CurrentConsumption) as CurrentConsumption,Convert(varchar,CurrentConsumptionValue) as CurrentConsumptionValue ,Convert(varchar,VATValue) as VATValue,
Convert(varchar,CleaningFees) as CleaningFees from WAHInvoice invo where 1=1 AND year=2018 AND  BranchCode = 1/* and Month=5  and UnitCode=5639*/
union
SELECT VTargetDetail.BranchCode ,VTargetDetail.SubLdgCode as UnitCode ,
(SUM(dbo.VTargetDetail.Debit * dbo.VTargetDetail.CurrencyRate)  -  SUM(dbo.VTargetDetail.Credit * dbo.VTargetDetail.CurrencyRate)  ) as Balance ,'' AS Serial ,'' as YEAR,'' AS Month,'' as CurrentReadingDate,'' as CurrentMeterReading,'' as LastMeterReading,'' as CurrentConsumption,'' as CurrentConsumptionValue,'' as VATValue,'' as CleaningFees
                                FROM         dbo.VTargetDetail LEFT OUTER JOIN
                              dbo.TypeTrxSafe ON dbo.VTargetDetail.BranchCode = dbo.TypeTrxSafe.BranchCode AND dbo.VTargetDetail.TrxTypeCode = dbo.TypeTrxSafe.SafeTrxTypeCode
                              WHERE     (1 = 1) AND (VTargetDetail.TrxDate <=  '2018/09/17') AND (VTargetDetail.SubLdgCodeType = 28) /*AND (VTargetDetail.SubLdgCode = 5639) */
                              group by VTargetDetail.SubLdgCode,VTargetDetail.BranchCode
                              )as INV where Month=5 and UnitCode=5639