Sales Credit Memo

Hi, I want to post a sales cr. memo from my own codeunit

of filtered sales invoice from (Cust. Ledger ENtry) which is already posted, 

sales posting is done by (Sales-Post) codeunit

now problem is this i am unable to sort out how to set filter the line from,

can any one tell about this structure, i am new on NAV

THANKS in advance