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