Not making Invoice in AP for Credit Memo of Account Receivables

 Hi All,

I am running a scenario of booking a sales order shipping the goods to the customer , the customer pays in full eg 1000$.

No some goods were damaged and were sent back by the customer ,  Return order was created for 300$ that particular sales order and the goods were recieved back in the inventory ,  a credit memo of 300$ is generated for that return in AR , by running auto Invoice , and the credit memo is also applied on the 1000$ invoice , now i have to refund the money back to the customer . I go to AP and enter an invoice for 300$ i click find button and select Payment Request for the Credit memo but it is