I am trying to create a credit memo for invoice having multiple lines which is partially paid.

I am trying to create a credit memo for invoice having multiple lines which is partially paid.  

Can anyone help me with this?

I am trying to insert these lines into interface table, but as this is at line level its either taking full invoice amount if i refer to unit selling price or if i take amount as remaining outstanding amount , it adds up the amount for number of lines.

Any leads will be appreciated