Random assigning of numbers in Postgres

Although, in theory, one could generate a bijective function between the domains 1..500000 and 9353500001234...9353500501234, in practice, the easiest way is just to create a map (in the mathematical sense), and SELECT from it.

That is, you would start by creating a table to store pairs of (employee_id, card_number):

CREATE TABLE employee_card
(
    employee_id INTEGER NOT NULL PRIMARY KEY,
    card_number BIGINT NOT NULL UNIQUE 
) ;

Given that table, we fill it with all possible employee_id and card_number, randomly shuffling the card_numbers. We do that by means of the generate_series set returning function, together with row_number(), using a random()ly sorted window (over clause):

INSERT INTO employee_card (employee_id, card_number)
SELECT
    row_number() over(order by random()) AS employee_id, employee_card
FROM
    -- generate card_numbers
    generate_series(9353500001234, 9353500501234) AS s(employee_card)

We add some indexes to get data in an optimal way:

-- Indexes to ease retrieving the data from this table
CREATE INDEX idx_employee_card_card_number ON employee_card(card_number, employee_id);
CREATE INDEX idx_employee_card_employee_id ON employee_card(employee_id, card_number);
VACUUM ANALYZE employee_card;

At this point, if you want to have two functions, they are just SELECTs from the previous table:

-- Get card_number given employee_id
CREATE OR REPLACE FUNCTION card_number_from_employee_id(_employee_id integer) 
    RETURNS BIGINT
    IMMUTABLE
    STRICT
    LANGUAGE SQL
AS
$$
    SELECT card_number FROM employee_card WHERE employee_id = _employee_id ;
$$ ;

-- Get employee_id given card_number
CREATE OR REPLACE FUNCTION employee_id_from_card_number(_card_number BIGINT)
    RETURNS INTEGER
    IMMUTABLE
    STRICT
    LANGUAGE SQL
AS
$$
    SELECT employee_id FROM employee_card WHERE card_number = _card_number ;
$$ ;

You can now test a few employee_ids, and retrieve card numbers. The card numbers should look random:

SELECT 
    employee_id, card_number_from_employee_id(employee_id)
FROM
    generate_series(10001, 10010) AS s(employee_id) ;

employee_id | card_number_from_employee_id
----------: | ---------------------------:
      10001 |                9353500008121
      10002 |                9353500281438
      10003 |                9353500124657
      10004 |                9353500369382
      10005 |                9353500238058
      10006 |                9353500205141
      10007 |                9353500297414
      10008 |                9353500286020
      10009 |                9353500219563
      10010 |                9353500443892

And also test the reverse function:

SELECT
    card_number, employee_id_from_card_number(card_number)
FROM
    generate_series(9353500001234 + 1023, 9353500001234 + 1023 + 9) AS s(card_number);

  card_number | employee_id_from_card_number
------------: | ---------------------------:
9353500002257 |                        46754
9353500002258 |                        25124
9353500002259 |                       481462
9353500002260 |                        99475
9353500002261 |                       173732
9353500002262 |                       171043
9353500002263 |                       245192
9353500002264 |                       418200
9353500002265 |                        76374
9353500002266 |                       251095

You can check all the setup and functions at dbfiddle here