Create table x_inv
( 
Inv_num Number,
Inv_date Date
);


Insert into x_inv values (400,sysdate);

Insert into x_inv values (300,sysdate-1);

Insert into x_inv values (200,sysdate-2);

Insert into x_inv values (100,sysdate-3);

SQL> with
  2    data as (
  3      select inv_num, inv_date, lag(inv_num) over (order by inv_date) prev_num
  4      from x_inv
  5    )
  6  select inv_num, inv_date
  7  from data
  8  where prev_num = &prev_num
  9  /
Enter value for prev_num: 300
   INV_NUM INV_DATE
---------- -------------------
       400 11/05/2016 13:21:18