When you are ready to process the payment, click the "Apply" button.  This will post the payment to the customers account and leave you in the enter cash receipt window.

 

If you click the "OK" button the payment will be posted to the customer's account, but the enter cash receipts window will be closed and you will be returned to the entrée main menu.

 

Clicking the "Cancel" button will discard the current payment and return you to the entrée main menu.