UPDATE T1
SET T1.Amount = (
    SELECT SUM(T2.Amount) * -1 
    FROM TblAccounting T2
    WHERE T2.VoucherNo = T1.VoucherNo 
      AND T2.AcCode = 10000
      AND T2.Id > T1.Id  -- Selecting rows "below" based on Id
)
FROM TblAccounting T1
WHERE T1.Amount is null and type='CustomerReceipt'