Payment Method of Invoice & payment work bench

Hi Experts,

                    I have Issue with payment Method where the Credit memo is not available in Payment work bench when clicking on Enter/Adjust Invoices.

Scenario:-

1. I have 3 Invoices to be paid. I.e ( 2-  Standard Invoices with Payment Method as PDC & One Credit Memo with Payment Method as Check defined in Invoice Work Bench)

2. When I select Payment Type as Quick/Manual & in Header of payment, I have mentioned Payment Method as PDC & when I click on Enter/Adjust Invoices in payment Work bench, am able to select 2- Standard Invoices but unable to see Credit memo. Why ?

     Now My question is, Can we make Payment for Invoice whose Payment Method is Check & while making Payment, Can we select the Payment Method as PDC in Payment work Bench. Please advise