Credit memo of an invoice with prepayment

zutzu

I post a prepayment of a purchase order.
I post the invoice of that order and there is an invoice line with negative quantity to discount the amount prepaid.
I want to make a credit memo of that invoice with the discount of the prepayment.
In credit memo form, in button "Functions, get posted document lines to reverse", select the invoice but doesn´t appear the line with the negative quantity.
Is this an error?

If i post an invoice with a line with negative quantity, if a make a credit memo and choose in button "Functions, get posted document lines to reverse", it makes a credit memo like the invoice.
But if i make a credit memo with an invoice with prepayment includes, the credit memo is not equal than the invoice, because there is no line with negative quantity.

Is there any solution?