On the form, need three elements: Beginning Balance, Payment, Ending Balance
Make Ending Balance ReadOnly.
If the Beginning Balance is imported, posted in, or calculated vs. manual data entry, then make the Beginning Balance ReadOnly also.

Put formula in Payment element, On Element Change
Ending Balance = Beginning Balance - Payment

------------------------
This seems basic to me, but perhaps I am not understanding the question.  Not sure by what you mean that you want to add a Payment line.  This added a Payment element.  Do you need to add a subform to the existing subform?  Or perhaps you want the ending balance to be on the Parent Form?

As a general concept, I try to make most elements ReadOnly that are the result of anything except manual entry.  That is to prevent accidental changing of something that may have been the result of other actions. 
I also make KeyID fields ReadOnly if not @ISNEW.  Prevents seeing Key.. values in that element when someone thinks they are on the Retrieve Screen and replaces original KeyID value.

After doing some reading i have change it so it will pop up an window with the transactions on it