Postgresql – 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.