I always emailed the invoice plus the transfer receipt. In addition, I wrote the amount after discount under the invoice, as long as it was not specified. It worked fine that way and there were never any inquiries.
We also specified the invoice amount that way. No one wanted to see transfer receipts, only proof of equity. When the submitted invoices added up so that the equity was almost used up, we made the first disbursements – mostly tranches of 50 or 100k€. For the next disbursement, I then sent the new batch of invoices again and that was it.