Check total amount owed for a user’s invoices

I have written months ago but this is not yet solved and it is urgent for us. There is a page in sliced invoices where you see all invoices as the admin. In here you can select the client and see all the invoices from that client, however you cannot see a total amount of money in the end. I would like to see how much this client owes as a total, rather then just adding up all of his invoices to calculate his / her total. Its just too tedious! Also below the total, there should be a discount option, where I add a discount for the total amount of money he/she owes instead of applying a discount in every invoice separately. And in the client area, were the client sees all of his invoices, there should also be a field in the table that shows discounts applied by admin right near the amount of that specific invoice. This is urgent for us, we are trying to solve it for months now but we cannot and it is causing us a problem. Is there a way to solve this or could you please include it in your next version?