variables in sql functions

Is there a way for something like that to be made with sql function
not plpgsql

CREATE FUNCTION resource_limit($int4, date) RETURNS BOOLEAN
AS '
SELECT SUM(amount) as $user_amount FROM user_resource WHERE id = $1;
SELECT SUM(amount) > ($user_amount) FROM resource WHERE enddate > $2
'
LANGUAGE 'sql'
WITH (isstrict);

It is a reduced version, but it gives the idea of what I need - I need
to select a constant for the duration of the function data as a
variable and use it for the later query.