Create Trigger on Oracle

I have to create a trigger that applies a 15% discount to any purchase made by the top client (the one with the highest total purchase amount among all the clients). But there are some compilation errors.

    CREATE OR REPLACE TRIGGER TOP_DISCOUNT
    BEFORE INSERT ON PURCHASE
    FOR EACH ROW
    DECLARE M_A_X NUMBER;
    BEGIN
    SELECT MAX(AMOUNT) INTO M_A_X FROM PURCHASE;
    SELECT (:NEW.AMOUNT * 0.85) INTO :NEW.AMOUNT FROM DUAL;
    WHEN AMOUNT IN M_A_X;
    END;
    /