This might at least help a bit, but it won't address the main problems of the query (user defined scalar function and function on columns in the where clause or others that we can't see).

select @QTY_cost = (select round(sum(TXN.Fn_bacashcalculation(TXN.Transaction_Code,

TXN.SEC_TYPE_CODE1,

TXN.Sec_Type_Code2,

TXN.TRADE_AMOUNT,

'Y') +

TXN.Fn_bacashcalculation(TXN.TRANSACTION_CODE,

TXN.SEC_TYPE_CODE1,

TXN.SEC_TYPE_CODE2,

TXN.TRADE_AMOUNT,

'N')), 0) as QTY

from TXN.TRANSACTION_DETAILS TXN with (nolock) --Why NOLOCK? Are you fine with inconsistent results?

where TXN.RUBEL_ID = @Rubel_ID

and TXN.SECURITY_ID2 = @Security_ID

and not (TXN.TRANSACTION_CODE in ('dp', 'wd')

and TXN.SECURITY_ID1 = TXN.SECURITY_ID2

)

and TRADE_DATE <= @Report_dt

and not (TXN.TRANSACTION_CODE in ('sa', 'pa')

and isnull(SETTLE_DATE, TRADE_DATE) > @Report_dt

)

and TXN.END_DT is null

);