This is insane LOL.  All of the questions saying the same thing, but nobody on support can even spend enough time to understand the problem, let alone offer a helpful response.  

I'm having the same issue.  I have a customer with an invoice.  I need them to pay 50% of the invoice up front.  I want to send them a link that asks for just 50% of the invoice (or other custom amount), but all I can do is send them the full invoice, which always causes confusion, leading me to explain how quickbooks doesn't allow me to do that.  

Your support staff say this is possible.  Nobody here seems to be able to get it to work.  Please, read this thoroughly, and then answer the question like a real human rather than a robot.