Unable to create credit for negative contract invoice line

Summary:

We have a requirement where we would like to create a partial credit for a contract invoice with multiple lines. There are both negative and positive invoice lines. However, only the positive invoice line allow us to input an amount to initiate a credit memo, the negative lines are greyed out and does not allow input. May I know how we can allow the negative invoice line to also allow input?

All event types already have "Allow Adjustment" selected.

Thanks!

Alan