Custom Amount on Invoice

To me, a really important feature for the invoice would be to have to ability to send an invoice with a specific amount due. For example if the invoice is 35k and they make the first deposit of 80%, then the invoice is updated to reflect add-ons as the project progresses, I would like to be able to send a request for the updated 80%. However as it is now, I can only get the initial deposit and the final amount. I would need to utilize an external program such as quick books to get a custom amount.