I don't think you got an answer for this. Quickbooks is really bad at some things and this is one area of such.
Here is what you do. I tried it and it works. You first deposit the prepayment into a liability account. You now have that money and you owe it. Cool. When you are ready to invoice the customer, first you need to transfer funds from a "fake" bank which I call "not for deposit." I use this for all sorts of things. You need to make this bank account for many types of transitory things and work arounds. Note that it must always be at zero after using it per the full process. Now you transfer money from that "fake" bank into the liability account. Now you have a negative balance in your not for deposit account and your liability was set back to zero since it got money from your bank account. Cool again. Now you make an invoice for your customer and he needs to pay it. How does he do that if he already paid it? Magic. That is how. You receive payment on that invoice into your fake account. The balance of your not for deposit account now goes back to zero because your customer just deposited fake money there. And his invoice is all paid up. No more Liability. No more nothing. It all works. yes i am a genius and I use these work arounds for other things as well.
