Need Total To Reset Inside A Trigger

Have the following trigger:

CREATE TRIGGER [UPDATEEGBKMUTREBATERECORDS] ON [dbo].[GBKMUT]
after INSERT
AS
begin
DECLARE@Sum Float
SELECT@Sum = SUM(bdr_hfl)
FROMinserted
WHEREfreefield3 = 'Rebate'

SET@Sum = COALESCE(@Sum, 0)
UPDATEgbkmut
SETbdr_hfl = bdr_hfl - @Sum
WHEREreknr = ' 1040'
end

I need @Sum to reset to zero if the ord_no changes. I'm inserting discount records that need to subtract from another account's amount.
As records are inserted for one ord_no and Inv_no I need to sum bdr_hfl field. When the ord_no and Inv_no change, I need to subtract that from the amount in the bdr_hfl where the account number is 1040 and the inserted records ord_no and Inv_no match the ord_no and Inv_no where the account is 1040. Then I need the @sum to reset to zero and start summing again.