When you're on the Apply Sales Documents screen if you click the look up glass next to the Document No. you will see all other credits for this customer (you may need to scroll up). If you choose one it will bring it onto the apply screen and let you apply it. You can keep doing this will all the previously posted credits, returns and cash receipts int he list.
However, you will need to then click OK on the Apply screen, go back to the Cash Receipts screen and re-enter the Apply screen to be able to finish applying the current cash receipt that you're entering.
The same functionality exists on the Apply Payables Documents screen.