think you need to setup a rule, ive got a payment processed rule that activates and displays the change on screen. let me upload a screen shot to see if its what your looking for
EDIT
EDIT
i dont use this next set up as i auto log out after every sale so i wouldnt see this but this is another way where you can display the change by using the shell feature and some actions and rules (think you need to download this add on from samba market)
See this link for more info Update Application Subtitle with LAST SALE