I changed the not null constraint but I dont insert the FK id (it is null)
drop table C;
DROP TABLE
s=> create table C (
"id" SERIAL PRIMARY KEY,
name varchar(40)
, b_id integer references B(id)DEFERRABLE INITIALLY DEFERRED);
CREATE TABLE
s=> insert into A (var1, var2) values ('Hello1', 'World1');
INSERT 0 1
s=> select * from C;
1 | World1 |