Postgresql, set order by desc or asc depending on variable parse into function

I have a function that takes product pricing data from today and yesterday and works out the difference, orders it by price_delta_percentage and then limits to 5. Now currently I order by price_delta_percentage DESC which returns the top 5 products that have increased in price since yesterday.

I would like to parse in a variable - sort - to change the function to either sort by DESC, or ASC. I have tried to use IF statements and get syntax errors and CASE statements which states that price_delta_percentage doesn't exist.

Script:

RETURNS TABLE(
    product_id varchar, 
    name varchar, 
    price_today numeric,
    price_yesterday numeric,
    price_delta numeric,
    price_delta_percentage numeric
)
LANGUAGE 'sql'
COST 100
STABLE STRICT PARALLEL SAFE 
AS $BODY$
    WITH cte AS (
        SELECT
            product_id,
            name,
            SUM(CASE WHEN rank = 1 THEN trend_price ELSE NULL END) price_today, 
            SUM(CASE WHEN rank = 2 THEN trend_price ELSE NULL END) price_yesterday,
            SUM(CASE WHEN rank = 1 THEN trend_price ELSE 0 END) - SUM(CASE WHEN rank = 2 THEN trend_price ELSE 0 END) as price_delta,
            ROUND(((SUM(CASE WHEN rank = 1 THEN trend_price ELSE NULL END) / SUM(CASE WHEN rank = 2 THEN trend_price ELSE NULL END) - 1) * 100), 2) as price_delta_percentage
        FROM (
            SELECT
                magic_sets_cards.name,
                pricing.product_id,
                pricing.trend_price, 
                pricing.date, 
                RANK() OVER (PARTITION BY product_id ORDER BY date DESC) AS rank
            FROM pricing
                JOIN magic_sets_cards_identifiers ON magic_sets_cards_identifiers.mcm_id = pricing.product_id
                JOIN magic_sets_cards ON magic_sets_cards.id = magic_sets_cards_identifiers.card_id
                JOIN magic_sets ON magic_sets.id = magic_sets_cards.set_id
            WHERE date BETWEEN CURRENT_DATE - days AND CURRENT_DATE
                AND magic_sets.code = set_code
                AND pricing.trend_price > 0.25) p
        WHERE rank IN (1,2)
        GROUP BY product_id, name
        ORDER BY price_delta_percentage DESC)
    SELECT * FROM cte WHERE (CASE WHEN price_today IS NULL OR price_yesterday IS NULL THEN 'NULL' ELSE 'VALID' END) !='NULL'
    LIMIT 5;
$BODY$;sql

CASE Statement:

ORDER BY CASE WHEN sort = 'DESC' THEN price_delta_percentage END DESC, CASE WHEN sort = 'ASC' THEN price_delta_percentage END ASC)

Error:

ERROR:  column "price_delta_percentage" does not exist
LINE 42:   ORDER BY CASE WHEN sort = 'DESC' THEN price_delta_percenta...