Vantage - A/R Problem -

In the past you have helped me with Vantage A/R problem - with my limit
experience in accounting - I need your help or advise. I

Anyways, my accounting person made a mistake. We had a sales order with
two line items. The terms are 50% down with order and then 50% before

She advance billed 50% down okay. (line item 1 she billed $2750 -
total item is 5500 - Second line item she billed correctly also 2100 -
total item is 4200).

We received payment and did cash receipts.

Now it is time to do the second adv. billing before shipment. (This is
were we screwed up) Line item one - she only adv. billed for $650.00
(short $2100) and Line item two she adv. billed $4200 - the entire
amount (over $2100). But it was not caught here. - she received payment
and posted.

Next we shipped the part. She is ready to do a zero balance invoice -
but because of the difference I can not do a zero balance.

Now - what I was going to do is - Enter a Advance Billing with a misc
charge for both of the items (the difference amount) which would give me
a zero balance invoice. Then recreate the advance billing. - then give
a credit memo. - Is there any problem with this? Or any other way?