Applying INVOICE Against CM

I created an manual invoice thru  Navigation Transactions> Transactions and selected the Class INVOICE with invoice number MAN123  amounting $500.   I have another transactions created thru Navigation Transactions> Transactions and selected the Class CREDIT MEMO with credit memo number CM123 amounting to $100  . Please note that these two transactions  belong to the same customer and created thru Transactions> Transaction.  My question is how can associate my Credit Memo to the Invoice so that when I  pull the invoice it will show the deduction  from Details of the Balance Due Region of the Invoice.