FB05 OR F-30

Hi

I enter t code FB05 and F-30.

I enter header information like company code etc.

I choose Incoming Payments radio button

I enter 40 G L Account and amount. I enter a value of '100' in assignment field

Now I want to choose Process Open Items

Now customer type K is proposed and I want to clear customer line item whose assignment line items is already populated with '100

Now I try to choose assignment radio button shown on the right side.

But system proposes only Document number and reference fields only.

Now If I want to use any othe selection criteria for clearing purpose what should I have to do?

Pl help me do this functionality as I need it very urgently.

Joe