Add foreign key constraint to an existing table column, referencing a column in a newly created table

Let’s say I have one table defined as this

CREATE TABLE sales (
    agent_id   integer references agents(agent_id),
    sale_date  date,
    amout      numeric(10,2)
);

Then an ETL process fills this table with data

INSERT INTO sales VALUES
    (1, '2019-01-01', 1031.11),
    (1, '2019-01-02', 525.44),
    (1, '2019-01-03', 323.99);

But later, I need to add a table with actual processed dates, such as

CREATE TABLE dates (
    date_idx   date primary key,
    year       integer not null,
    month      integer not null,
    day        integer not null
);

and want to add a foreign key constraint on the old table:

ALTER TABLE sales
    ADD CONSTRAINT sales_date_fk FOREIGN KEY (sale_date)
    REFERENCES dates (date_idx);

Naturally, I get the following error:

ERROR: insert or update on table “sales” violates foreign key constraint “sales_date_fk” Detail: Key (sale_date)=(2019-01-01) is not present in table “dates”.

I know I can work around this by deleting/truncating all data in sales or prefilling dates before adding the constraint, but I would prefer not to if I can avoid it.

Is there any way to accomplish this?

I’m using Postgres 11, but this fiddle running 9.6 shows what I’ve tried to explain here.

ALTER TABLE sales
ADD CONSTRAINT sales_date_fk FOREIGN KEY (sale_date)
REFERENCES dates (date_idx) NOT VALID;