Oracle/SQL Find Date or Closest earlier date to specified date

I am trying to run a query within oracle which creates a value. The underlying table does not always contain a match for a specific date. So i have to either find the record with the date specified or find the closest date below the specified date.

SUM(CASE 
            WHEN S.VAL_SECURITY NOT IN ('GBP')
                AND E.SECURITY_ID NOT IN ('GBP')
                THEN (
                        (((amount) * - 1) / nvl(s.minor_per_unit, 1)) * (
                            SELECT EXCHANGE_RATE
                            FROM ISET.PREVIOUS_PRICES
                            WHERE ISET.PREVIOUS_PRICES.SECURITY_ID = E.SECURITY_ID
                                AND VAL_DATE = TO_DATE('05/08/2019', 'DD/MM/YYYY')
                            )
                        ) * (
                        SELECT EXCHANGE_RATE
                        FROM ISET.PREVIOUS_PRICES
                        WHERE SECURITY_ID = S.VAL_SECURITY
                            AND VAL_DATE = TO_DATE('05/08/2019', 'DD/MM/YYYY')
                        )
            WHEN S.VAL_SECURITY IN ('GBP')
                AND E.SECURITY_ID NOT IN ('GBP')
                THEN (
                        (((amount) * - 1) / nvl(s.minor_per_unit, 1)) * (
                            SELECT EXCHANGE_RATE
                            FROM ISET.PREVIOUS_PRICES
                            WHERE ISET.PREVIOUS_PRICES.SECURITY_ID = E.SECURITY_ID
                                AND VAL_DATE = TO_DATE('05/08/2019', 'DD/MM/YYYY')
                            )
                        )
            ELSE ((amount) * - 1) / nvl(s.minor_per_unit, 1)
END) GBP_VALUE

The code in question is

E.SECURITY_ID AND VAL_DATE = TO_DATE('05/08/2019', 'DD/MM/YYYY')

So assuming it doesn't find a record for the specified date then it needs to find the max date record closest to but below the specified date.

Any help would be greatly appreciated.

Thank you

Could you try this? I haven't verified it.

    val_date =
(SELECT val_date
   FROM (SELECT MAX (b.val_date) val_date,
                MIN (TO_DATE ('05/08/2019', 'DD/MM/YYYY') - b.val_date)
                   diff
           FROM PREVIOUS_PRICES b
          WHERE     SECURITY_ID = E.SECURITY_ID
                AND status_date <= TO_DATE ('05/08/2019', 'DD/MM/YYYY')
         UNION ALL
         SELECT MIN (val_date),
                MIN (b.val_date - TO_DATE ('05/08/2019', 'DD/MM/YYYY'))
           FROM PREVIOUS_PRICES b
          WHERE     SECURITY_ID = E.SECURITY_ID
                AND status_date > TO_DATE ('05/08/2019', 'DD/MM/YYYY')
         ORDER BY 2)
  WHERE ROWNUM = 1)