That is something we can work on. We may need to ask for a little more info and maybe testing feedback, but we're on our way at least.
I'm not sure here where aggregate functions come in and what they can reference correctly without access to an example, which I don't have to hand. I know subforms can be freferenced in a number of ways. Your example will not work as posted. This may be for various reasons - The most likely of which is that you haven't copied your code across accurately. Workable ways are shown in
Referring to Items on a Sub-Form. I suggest you copy and paste the actual formula you have at this time. I'd also be interested to see the actual formula for something that correctly references the controls, even though it doesn't manage to aggregate or sum them.
What we need to look at first is whether a control outside of your subform can aggregate (EG.
Sum()) the values referenced in the controls found within the subform ([Charge Amt] & [Pymnt Amnt]). I suspect not, but let's get a test done to confirm before proceeding.
If it cannot, then the next thing to try would be to create a new textBox control in the subform's footer section (Ensure Form Header/Footer visible from the View Menu). This control may be hidden if preferred. The formula sums the subtraction of the two controls in the Detail section (
=Sum([Charge Amt]-[Pymnt Amnt])). All this is within the subform at this stage. For the purposes of reference, we'll call this control [txtTotProfit].
This new control ([txtTotProfit]) should now be referenced directly by the control you are currently trying to set up to show the value on the main form.