Did Credit/rebill with adj entire bill page, on standard billing page when I pull up the rebill won'

When I do a credit and rebill on the adjust entire bill page, I look up the rebill on the standard billing page and the page won't allow me to change the bill if the amount is a credit. On the rebill,  I am trying to split the amount into to credit amounts on two different lines with two different projects.  I get the following message: 'Cannot modify this bill line. Credit adjustment lines (reversal of line charges) cannot be modified (1200,126).  Credit only bills and /or bill lines are meant to exactly reverse already invoiced bills in order to