SQL> create table test (due_amt number, due_amt_sgn varchar2(1));

Table created.

SQL> 

load data
  infile *
  replace
into table test
  fields terminated by ','
  trailing nullcols
(due_amt "decode(:due_amt_sgn, '-', -:due_amt, :due_amt)",
 due_amt_sgn
)

begindata
100,+
200,-
300,-
400,+

SQL> $sqlldr scott/tiger control=test.ctl log=test.log

SQL*Loader: Release 10.2.0.1.0 - Production on Sri Lip 17 22:43:14 2009

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

Commit point reached - logical record count 3
Commit point reached - logical record count 4

SQL> select * from test;

   DUE_AMT D
---------- -
       100 +
      -200 -
      -300 -
       400 +

SQL>