Generating first invoice from PrePaid hours

Hey everyone,
I did some initial searching around and didn’t find anything, hopefully i’m not duplicating work here. But i setup a client with prepaid hours, and i see it fine in their dashboard, but now i’m trying to invoice them for the work done, and top up those pre paid hours. When i generate the invoice however it just charges them for all the work done and doesn’t subtract anything from the prepaid hours. I am using correct line items, meaning, all my hours are under “Labor”. Not sure if i did anything wrong generating the tickets or if I missed something else. Was wondering if anyone could help me out on how this process should go?