need to take a sum of a sum in a query

Hi all, I have this query

Select
  a.TERM,
  a.STUDENT_ID,
  a.AR_TYPE,
  Sum(a.INVOICE_AMT) - Sum(a.PAYMENT_AMT) As TERM_BALANCE,
  STUDENT_CURRENT_TYPE_VIEW.STUDENT_CURRENT_TYPE,
  b.STA_CLASS,
  PREFERRED_MAIL_VIEW.[First Name],
  PREFERRED_MAIL_VIEW.[Last Name],
  PREFERRED_MAIL_VIEW.PERSON_STATUS,
  PREFERRED_MAIL_VIEW.[Address Line 1],
  PREFERRED_MAIL_VIEW.[Address Line 2],
  PREFERRED_MAIL_VIEW.[Address Line 3],
  PREFERRED_MAIL_VIEW.City,
  PREFERRED_MAIL_VIEW.State,
  PREFERRED_MAIL_VIEW.Zip,
  PREFERRED_MAIL_VIEW.Country,
  PREFERRED_MAIL_VIEW.[Preferred Email],
  TERMS.TERM_START_DATE
From
  (Select
    AR_INVOICES.INV_TERM As TERM,
    AR_INVOICES.INV_PERSON_ID As STUDENT_ID,
    AR_INVOICES.INV_AR_TYPE As AR_TYPE,
    0 As PAYMENT_AMT,
    IsNull(AR_INVOICE_ITEMS.INVI_EXT_CHARGE_AMT, 0) -
    IsNull(AR_INVOICE_ITEMS.INVI_EXT_CR_AMT, 0) As INVOICE_AMT
  From
    AR_INVOICE_ITEMS Inner Join
    AR_INVOICES On AR_INVOICE_ITEMS.INVI_INVOICE = AR_INVOICES.AR_INVOICES_ID
  Where
    AR_INVOICES.INV_TERM Is Not Null
  Union All
  Select
    IsNull(AR_PAYMENTS.ARP_TERM, AR_INVOICES.INV_TERM) As TERM,
    AR_PAYMENTS.ARP_PERSON_ID,
    AR_PAYMENTS.ARP_AR_TYPE,
    IsNull(AR_PAYMENT_ITEMS.ARPI_AMT, 0) -
    IsNull(AR_PAYMENT_ITEMS.ARPI_REVERSAL_AMT, 0) As PAYMENT_AMT,
    0 As INVOICE_AMT
  From
    AR_PAYMENT_ITEMS Inner Join
    AR_PAYMENTS On AR_PAYMENT_ITEMS.ARPI_PAYMENT = AR_PAYMENTS.AR_PAYMENTS_ID
    Left Outer Join
    AR_INVOICE_ITEMS On AR_PAYMENT_ITEMS.ARPI_INVOICE_ITEM =
      AR_INVOICE_ITEMS.AR_INVOICE_ITEMS_ID Left Outer Join
    AR_INVOICES On AR_INVOICE_ITEMS.INVI_INVOICE = AR_INVOICES.AR_INVOICES_ID
  Where
    IsNull(AR_PAYMENTS.ARP_TERM, AR_INVOICES.INV_TERM) Is Not Null) As a
  Left Join
  STUDENT_CURRENT_TYPE_VIEW
    On a.STUDENT_ID = STUDENT_CURRENT_TYPE_VIEW.STUDENTS_ID Left Join
  (Select
    *
  From
    STUDENT_ACAD_LEVELS_VIEW
  Where
    STUDENT_ACAD_LEVELS_VIEW.STA_ACAD_LEVEL = 'FP') As b On a.STUDENT_ID =
    b.STA_STUDENT Left Join
  PREFERRED_MAIL_VIEW On a.STUDENT_ID = PREFERRED_MAIL_VIEW.ID Left Join
  TERMS On TERMS.TERMS_ID = a.TERM
Group By
  a.TERM, a.STUDENT_ID, a.AR_TYPE,
  STUDENT_CURRENT_TYPE_VIEW.STUDENT_CURRENT_TYPE, b.STA_CLASS,
  PREFERRED_MAIL_VIEW.[First Name], PREFERRED_MAIL_VIEW.[Last Name],
  PREFERRED_MAIL_VIEW.PERSON_STATUS, PREFERRED_MAIL_VIEW.[Address Line 1],
  PREFERRED_MAIL_VIEW.[Address Line 2], PREFERRED_MAIL_VIEW.[Address Line 3],
  PREFERRED_MAIL_VIEW.City, PREFERRED_MAIL_VIEW.State, PREFERRED_MAIL_VIEW.Zip,
  PREFERRED_MAIL_VIEW.Country, PREFERRED_MAIL_VIEW.[Preferred Email],
  TERMS.TERM_START_DATE
Having
  a.STUDENT_ID Is Not Null
Order By
  a.STUDENT_ID

And basically what I need to add is a total balance for all terms for a student prior to the current term. I am doing this in SSRS so I was thinking of creating a parameter of current term start date and calculating the total balance prior to the specified current term start date. So a balance for terms