My first thought would be to have a Table1:currenttotal field in the table shown in your first layout. Then from your second layout have a script Set Field Table1:currenttotal to Table2:runningtotal. You can put a button next to the running total on your second layout that runs that script and returns you to the first layout or make the field a button so all you have to do is click on that field.
You can define a calculation field in Businesses as Sum ( Payments::PaymentAmount ) so that you can place it in the portal to Businesses to report the total payments from the payment total for that business.
Your button that takes you from a contracts layout to the businesses layout can use this script step:
Go To Related Record [Show only related records; From table: Businesses; Using layout: "Businesses" (Businesses)]