How do i track payments and update the new balance due?

We sell each customer one item and most of them make payments on it till it is paid off.

We take in a CSV for each job that gives the DB the full total. the down payment they paid and the current balance due.

Here is how it shows in Ninox - the permit fee is allready included in the total.

Here is a screen shot of the invoice area i am calling Payments

Then here is a screen grab of the invoice payment print out look i made.

?

How do i set this up so it tracks all the new payments for each customer. It seems it will do this for each new invoice created but i need to update the new blance due each time some how.

So someplace in the customers info area i see a new field maybe called ( New Balance Due )  that shows the new updated total due after each payment.

And then i can pull this fields info in the invoice print out so each new payment they make i can send a new invoice/Receipt with the new updated amount due?

Right now i just have a blank area on the invoice for the new balance as i am not sure how to make that work?

I added in the new Number field so it is in place.

Thanks