Cash discount setup

Dear experts,

I have a question on how to setup cash discounts:

Scenario:

With our customer we have agreed that invoices are due for payment 30 days after the posting date. Should we receive payment within 10 days, a cash discount of 10% will be applied. If the payment is received between 11 and 20 days after issuing the invoice, a cash discount of 5% will be taken into consideration.

For an invoice with a posting date of 01.04.2010 the cash discount calculation is thus:

10% if payment received by 11.04.2010

5% if payment received by 21.04.2010

I have setup in BP -> Payment terms -> cash discount

Then when i go to incoming payments to perform an entry, the cash discount column is not updated. Pls advise if this way is correct?

thanks in advance.

Jo