Service item with discount amount not getting billed in invoice in sales order

Hi there is an scenario where user has created a sales order in which user has added a shippable item and non shippable item (service item) with a discount amount attached to the service item ( edit price) . So when the first line with shippable item is billed and closed , then later the service item line is brought to Awaiting Billing , but this is not getting invoiced. it is only stuck in Awaiting billing.

In the first case, when invoice ess job is run to invoice only the service item line with discount, it is giving the error message -

The transaction created can't have a negative amount because the creation sign of the transaction type is positive.