Foreign keys: how to turn referential integrity constraint off

I have a table of line_items and distributor_orders somewhat like this:

create table DIST_ORDER (

id serial primary key,
distributor_id integer references DISTRIBUTORS(id),
submit_time timestamp (0) without time zone not null
);

create table MEMBER_INVOICE_LI (

invoice_id integer references INVOICE_CART(id),
id integer not null, --line item number
dist_order_id integer references DIST_ORDER(id),
prod_id char(12) references PRODUCTS(id),
quantity int2 not null,
price integer not null,
shipped boolean not null,

primary key (invoice_id, id)
);

The problem I have is that distributor orders are only generated at the
end of the day, so when a customer creates an order, a line item is
created *but* there is no distributor order to assign it to yet.

How can I set the constraint to check referential integrity or let the
value be null?

Is this possible? Is it recommendable?

Thanks

Jc