I've seen a lot of good ideas mentioned above, but each of them have their own problems. Progress invoicing makes inventory management messy. Line items for partial payments reduce the clients open balance and subsequently your A/R reports. The line items used as payment term descriptions seems like a good way to go, but I believe the best method for managing 50/50 payment terms is simply to change the due date. The initial invoice has a due date set to whenever you need the deposit. This is between you and your client. It can be same day, it can be 1 year out. Doesn't matter. If the client wants to pay in full instead of half down, perfect!
However, if the client pays half down as you have requested, then go into the invoice, change the due date for when the remainder of the invoice is due, and resend the invoice to them. This will show them a reduction in the amount due (by however much they paid) while at the same time showing in your A/R that the remaining balance of their invoice isn't due yet.
I know this doesn't perfectly address the OP or some of the commenters, but considering the limitations of the system, this is the best way that I have found.
