Discount on one billing in Billing Plan

Hi All,

I have created a MV contract with a year and billed monthlly(01.01.2014-31.12.2014  $1000)

As a result, we will have the following item in billing plan.

01.01.2014 - 31.01.2014    $1000

01.02.2014 - 28.02.2014    $1000

01.03.2014 - 31.03.2014    $1000

etc.

And  I just want to have a discount condition on March billing, that is

01.01.2014 - 31.01.2014    $1000

01.02.2014 - 28.02.2014    $1000

01.03.2014 - 31.03.2014    $800(discount)

01.04.2014 - 30.04.2014    $1000

etc.

How can we achieve it? Since if i enter a discount condition  in Conditions Tab, it will apply to all the billing.

Thanks in advance.

David Tai