expression is testing fine, but value is not showing up in the app

[SANDRALAICHE]

I know this is going to be something simple, but I have been stumped for hours, I have tried so many different select formulas and have now just created a virtual column to first make a list of the values from the other table instead, I am trying to do a simple Balance Due Column that subtracts the list of payment amounts from the Invoice Amount Column, please help:

I have checked that all of my data types that I am creating the list from are all Price.

Thanks