Hi Samantha
I did try it and it creates planning lines in -ve qty as required.
Job Task No. Line Type Planning Date Planned Delivery Date Document No. Type No. Description Quantity Unit Price Line Amount
300 Billable 27/01/2021 27/01/2021 5678 G/L Account 6610 Sales, Other Job Expenses 1. 1,000.00 1,000.00
300 Billable 27/01/2021 27/01/2021 5678 G/L Account 6610 Sales, Other Job Expenses -1. 1,000.00 -1,000.00