Although it seems like a simple request, the way your data is structured makes it impossible. You are leveraging Measure Names and Measure Values which are handy but block the required table calculations.
You're going to need to pivot your data to split your original data rows into multiple rows each, one row for the Cash measure, one row for the Mortgage measure. That will take some trial and error but can be done in Tableau. Here's a link to get you started.
Then you can achieve what you want. An example is depicted and a TWBX is attached as well.
pivot data 104.twbx 945.1 KB